Simplify datetime parsing

This code as it stands pulls in the full datetime library from boost,
including io, and a bunch of timezone code.  The bmc doesn't make use of
any of this, so we can rely on a much simplified version.

Unfortunately for us, gcc still doesn't implement the c++20
std::chrono::parse[1].  There is a reference library available from [2]
that backports the parse function to compilers that don't yet support
it, and is the basis for the libc++ version.  This commit opts to copy
in the header as-written, under the assumption that we will never need
to pull in new versions of this library, and will move to the std
ersion as soon as it's available in the next gcc version.

This commit simplifies things down to improve compile times and binary
size.  It saves ~22KB of compressed binary size, or about 3%.

Tested: Unit tests pass.  Pretty good coverage.

[1] https://en.cppreference.com/w/cpp/chrono/parse
[2] https://github.com/HowardHinnant/date/blob/master/include/date/date.h

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I706b91cc3d9df3f32068125bc47ff0c374eb8d87

5 files changed, 8302 insertions, 30 deletions

diff --git a/meson.build b/meson.build
index fcc0d7c43e..9f22f23370 100644
--- a/meson.build
+++ b/meson.build
@@ -348,6 +348,7 @@ fs = import('fs')
 srcfiles_bmcweb = files(
   'redfish-core/src/error_messages.cpp',
   'redfish-core/src/registries.cpp',
+  'redfish-core/src/utils/time_utils.cpp',
   'redfish-core/src/utils/json_utils.cpp',
   'src/boost_asio_ssl.cpp',
   'src/boost_asio.cpp',
diff --git a/redfish-core/include/utils/extern/date.h b/redfish-core/include/utils/extern/date.h
new file mode 100644
index 0000000000..209e26774f
--- /dev/null
+++ b/redfish-core/include/utils/extern/date.h
@@ -0,0 +1,8254 @@
+// NOLINTBEGIN
+// clang-format off
+
+/*
+ *  Originally sourced from
+ *  https://github.com/HowardHinnant/date/blob/master/include/date/date.h
+ */
+
+#ifndef DATE_H
+#define DATE_H
+
+// The MIT License (MIT)
+//
+// Copyright (c) 2015, 2016, 2017 Howard Hinnant
+// Copyright (c) 2016 Adrian Colomitchi
+// Copyright (c) 2017 Florian Dang
+// Copyright (c) 2017 Paul Thompson
+// Copyright (c) 2018, 2019 Tomasz Kamiński
+// Copyright (c) 2019 Jiangang Zhuang
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+//
+// Our apologies.  When the previous paragraph was written, lowercase had not yet
+// been invented (that would involve another several millennia of evolution).
+// We did not mean to shout.
+
+#ifndef HAS_STRING_VIEW
+#  if __cplusplus >= 201703 || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
+#    define HAS_STRING_VIEW 1
+#  else
+#    define HAS_STRING_VIEW 0
+#  endif
+#endif  // HAS_STRING_VIEW
+
+#include <cassert>
+#include <algorithm>
+#include <cctype>
+#include <chrono>
+#include <climits>
+#include <cmath>
+#include <cstddef>
+#include <cstdint>
+#include <cstdlib>
+#include <ctime>
+#include <ios>
+#include <istream>
+#include <iterator>
+#include <limits>
+#include <locale>
+#include <memory>
+#include <ostream>
+#include <ratio>
+#include <sstream>
+#include <stdexcept>
+#include <string>
+#if HAS_STRING_VIEW
+# include <string_view>
+#endif
+#include <utility>
+#include <type_traits>
+
+
+#ifdef __GNUC__
+# pragma GCC diagnostic push
+# if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ > 7)
+#  pragma GCC diagnostic ignored "-Wpedantic"
+#  pragma GCC diagnostic ignored "-Wconversion"
+#  pragma GCC diagnostic ignored "-Wold-style-cast"
+# endif
+# if __GNUC__ < 5
+   // GCC 4.9 Bug 61489 Wrong warning with -Wmissing-field-initializers
+#  pragma GCC diagnostic ignored "-Wmissing-field-initializers"
+# endif
+#endif
+
+#ifdef __clang__
+#  pragma GCC diagnostic ignored "-Wunsafe-buffer-usage"
+#  pragma GCC diagnostic ignored "-Wzero-as-null-pointer-constant"
+#  pragma GCC diagnostic ignored "-Wold-style-cast"
+#endif
+
+#ifdef _MSC_VER
+#   pragma warning(push)
+// warning C4127: conditional expression is constant
+#   pragma warning(disable : 4127)
+#endif
+
+namespace date
+{
+
+//---------------+
+// Configuration |
+//---------------+
+
+#ifndef ONLY_C_LOCALE
+#  define ONLY_C_LOCALE 1
+#endif
+
+#if defined(_MSC_VER) && (!defined(__clang__) || (_MSC_VER < 1910))
+// MSVC
+#  ifndef _SILENCE_CXX17_UNCAUGHT_EXCEPTION_DEPRECATION_WARNING
+#    define _SILENCE_CXX17_UNCAUGHT_EXCEPTION_DEPRECATION_WARNING
+#  endif
+#  if _MSC_VER < 1910
+//   before VS2017
+#    define CONSTDATA const
+#    define CONSTCD11
+#    define CONSTCD14
+#    define NOEXCEPT _NOEXCEPT
+#  else
+//   VS2017 and later
+#    define CONSTDATA constexpr const
+#    define CONSTCD11 constexpr
+#    define CONSTCD14 constexpr
+#    define NOEXCEPT noexcept
+#  endif
+
+#elif defined(__SUNPRO_CC) && __SUNPRO_CC <= 0x5150
+// Oracle Developer Studio 12.6 and earlier
+#  define CONSTDATA constexpr const
+#  define CONSTCD11 constexpr
+#  define CONSTCD14
+#  define NOEXCEPT noexcept
+
+#elif __cplusplus >= 201402
+// C++14
+#  define CONSTDATA constexpr const
+#  define CONSTCD11 constexpr
+#  define CONSTCD14 constexpr
+#  define NOEXCEPT noexcept
+#else
+// C++11
+#  define CONSTDATA constexpr const
+#  define CONSTCD11 constexpr
+#  define CONSTCD14
+#  define NOEXCEPT noexcept
+#endif
+
+#ifndef HAS_UNCAUGHT_EXCEPTIONS
+#  if __cplusplus >= 201703 || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
+#    define HAS_UNCAUGHT_EXCEPTIONS 1
+#  else
+#    define HAS_UNCAUGHT_EXCEPTIONS 0
+#  endif
+#endif  // HAS_UNCAUGHT_EXCEPTIONS
+
+#ifndef HAS_VOID_T
+#  if __cplusplus >= 201703 || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
+#    define HAS_VOID_T 1
+#  else
+#    define HAS_VOID_T 0
+#  endif
+#endif  // HAS_VOID_T
+
+// Protect from Oracle sun macro
+#ifdef sun
+#  undef sun
+#endif
+
+// Work around for a NVCC compiler bug which causes it to fail
+// to compile std::ratio_{multiply,divide} when used directly
+// in the std::chrono::duration template instantiations below
+namespace detail {
+template <typename R1, typename R2>
+using ratio_multiply = decltype(std::ratio_multiply<R1, R2>{});
+
+template <typename R1, typename R2>
+using ratio_divide = decltype(std::ratio_divide<R1, R2>{});
+}  // namespace detail
+
+//-----------+
+// Interface |
+//-----------+
+
+// durations
+
+using days = std::chrono::duration
+    <int, detail::ratio_multiply<std::ratio<24>, std::chrono::hours::period>>;
+
+using weeks = std::chrono::duration
+    <int, detail::ratio_multiply<std::ratio<7>, days::period>>;
+
+using years = std::chrono::duration
+    <int, detail::ratio_multiply<std::ratio<146097, 400>, days::period>>;
+
+using months = std::chrono::duration
+    <int, detail::ratio_divide<years::period, std::ratio<12>>>;
+
+// time_point
+
+template <class Duration>
+    using sys_time = std::chrono::time_point<std::chrono::system_clock, Duration>;
+
+using sys_days    = sys_time<days>;
+using sys_seconds = sys_time<std::chrono::seconds>;
+
+struct local_t {};
+
+template <class Duration>
+    using local_time = std::chrono::time_point<local_t, Duration>;
+
+using local_seconds = local_time<std::chrono::seconds>;
+using local_days    = local_time<days>;
+
+// types
+
+struct last_spec
+{
+    explicit last_spec() = default;
+};
+
+class day;
+class month;
+class year;
+
+class weekday;
+class weekday_indexed;
+class weekday_last;
+
+class month_day;
+class month_day_last;
+class month_weekday;
+class month_weekday_last;
+
+class year_month;
+
+class year_month_day;
+class year_month_day_last;
+class year_month_weekday;
+class year_month_weekday_last;
+
+// date composition operators
+
+CONSTCD11 year_month operator/(const year& y, const month& m) NOEXCEPT;
+CONSTCD11 year_month operator/(const year& y, int          m) NOEXCEPT;
+
+CONSTCD11 month_day operator/(const day& d, const month& m) NOEXCEPT;
+CONSTCD11 month_day operator/(const day& d, int          m) NOEXCEPT;
+CONSTCD11 month_day operator/(const month& m, const day& d) NOEXCEPT;
+CONSTCD11 month_day operator/(const month& m, int        d) NOEXCEPT;
+CONSTCD11 month_day operator/(int          m, const day& d) NOEXCEPT;
+
+CONSTCD11 month_day_last operator/(const month& m, last_spec) NOEXCEPT;
+CONSTCD11 month_day_last operator/(int          m, last_spec) NOEXCEPT;
+CONSTCD11 month_day_last operator/(last_spec, const month& m) NOEXCEPT;
+CONSTCD11 month_day_last operator/(last_spec, int          m) NOEXCEPT;
+
+CONSTCD11 month_weekday operator/(const month& m, const weekday_indexed& wdi) NOEXCEPT;
+CONSTCD11 month_weekday operator/(int          m, const weekday_indexed& wdi) NOEXCEPT;
+CONSTCD11 month_weekday operator/(const weekday_indexed& wdi, const month& m) NOEXCEPT;
+CONSTCD11 month_weekday operator/(const weekday_indexed& wdi, int          m) NOEXCEPT;
+
+CONSTCD11 month_weekday_last operator/(const month& m, const weekday_last& wdl) NOEXCEPT;
+CONSTCD11 month_weekday_last operator/(int          m, const weekday_last& wdl) NOEXCEPT;
+CONSTCD11 month_weekday_last operator/(const weekday_last& wdl, const month& m) NOEXCEPT;
+CONSTCD11 month_weekday_last operator/(const weekday_last& wdl, int          m) NOEXCEPT;
+
+CONSTCD11 year_month_day operator/(const year_month& ym, const day& d) NOEXCEPT;
+CONSTCD11 year_month_day operator/(const year_month& ym, int        d) NOEXCEPT;
+CONSTCD11 year_month_day operator/(const year& y, const month_day& md) NOEXCEPT;
+CONSTCD11 year_month_day operator/(int         y, const month_day& md) NOEXCEPT;
+CONSTCD11 year_month_day operator/(const month_day& md, const year& y) NOEXCEPT;
+CONSTCD11 year_month_day operator/(const month_day& md, int         y) NOEXCEPT;
+
+CONSTCD11
+    year_month_day_last operator/(const year_month& ym,   last_spec) NOEXCEPT;
+CONSTCD11
+    year_month_day_last operator/(const year& y, const month_day_last& mdl) NOEXCEPT;
+CONSTCD11
+    year_month_day_last operator/(int         y, const month_day_last& mdl) NOEXCEPT;
+CONSTCD11
+    year_month_day_last operator/(const month_day_last& mdl, const year& y) NOEXCEPT;
+CONSTCD11
+    year_month_day_last operator/(const month_day_last& mdl, int         y) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday
+operator/(const year_month& ym, const weekday_indexed& wdi) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday
+operator/(const year&        y, const month_weekday&   mwd) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday
+operator/(int                y, const month_weekday&   mwd) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday
+operator/(const month_weekday& mwd, const year&          y) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday
+operator/(const month_weekday& mwd, int                  y) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday_last
+operator/(const year_month& ym, const weekday_last& wdl) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday_last
+operator/(const year& y, const month_weekday_last& mwdl) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday_last
+operator/(int         y, const month_weekday_last& mwdl) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday_last
+operator/(const month_weekday_last& mwdl, const year& y) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday_last
+operator/(const month_weekday_last& mwdl, int         y) NOEXCEPT;
+
+// Detailed interface
+
+// day
+
+class day
+{
+    unsigned char d_;
+
+public:
+    day() = default;
+    explicit CONSTCD11 day(unsigned d) NOEXCEPT;
+
+    CONSTCD14 day& operator++()    NOEXCEPT;
+    CONSTCD14 day  operator++(int) NOEXCEPT;
+    CONSTCD14 day& operator--()    NOEXCEPT;
+    CONSTCD14 day  operator--(int) NOEXCEPT;
+
+    CONSTCD14 day& operator+=(const days& d) NOEXCEPT;
+    CONSTCD14 day& operator-=(const days& d) NOEXCEPT;
+
+    CONSTCD11 explicit operator unsigned() const NOEXCEPT;
+    CONSTCD11 bool ok() const NOEXCEPT;
+};
+
+CONSTCD11 bool operator==(const day& x, const day& y) NOEXCEPT;
+CONSTCD11 bool operator!=(const day& x, const day& y) NOEXCEPT;
+CONSTCD11 bool operator< (const day& x, const day& y) NOEXCEPT;
+CONSTCD11 bool operator> (const day& x, const day& y) NOEXCEPT;
+CONSTCD11 bool operator<=(const day& x, const day& y) NOEXCEPT;
+CONSTCD11 bool operator>=(const day& x, const day& y) NOEXCEPT;
+
+CONSTCD11 day  operator+(const day&  x, const days& y) NOEXCEPT;
+CONSTCD11 day  operator+(const days& x, const day&  y) NOEXCEPT;
+CONSTCD11 day  operator-(const day&  x, const days& y) NOEXCEPT;
+CONSTCD11 days operator-(const day&  x, const day&  y) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const day& d);
+
+// month
+
+class month
+{
+    unsigned char m_;
+
+public:
+    month() = default;
+    explicit CONSTCD11 month(unsigned m) NOEXCEPT;
+
+    CONSTCD14 month& operator++()    NOEXCEPT;
+    CONSTCD14 month  operator++(int) NOEXCEPT;
+    CONSTCD14 month& operator--()    NOEXCEPT;
+    CONSTCD14 month  operator--(int) NOEXCEPT;
+
+    CONSTCD14 month& operator+=(const months& m) NOEXCEPT;
+    CONSTCD14 month& operator-=(const months& m) NOEXCEPT;
+
+    CONSTCD11 explicit operator unsigned() const NOEXCEPT;
+    CONSTCD11 bool ok() const NOEXCEPT;
+};
+
+CONSTCD11 bool operator==(const month& x, const month& y) NOEXCEPT;
+CONSTCD11 bool operator!=(const month& x, const month& y) NOEXCEPT;
+CONSTCD11 bool operator< (const month& x, const month& y) NOEXCEPT;
+CONSTCD11 bool operator> (const month& x, const month& y) NOEXCEPT;
+CONSTCD11 bool operator<=(const month& x, const month& y) NOEXCEPT;
+CONSTCD11 bool operator>=(const month& x, const month& y) NOEXCEPT;
+
+CONSTCD14 month  operator+(const month&  x, const months& y) NOEXCEPT;
+CONSTCD14 month  operator+(const months& x,  const month& y) NOEXCEPT;
+CONSTCD14 month  operator-(const month&  x, const months& y) NOEXCEPT;
+CONSTCD14 months operator-(const month&  x,  const month& y) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const month& m);
+
+// year
+
+class year
+{
+    short y_;
+
+public:
+    year() = default;
+    explicit CONSTCD11 year(int y) NOEXCEPT;
+
+    CONSTCD14 year& operator++()    NOEXCEPT;
+    CONSTCD14 year  operator++(int) NOEXCEPT;
+    CONSTCD14 year& operator--()    NOEXCEPT;
+    CONSTCD14 year  operator--(int) NOEXCEPT;
+
+    CONSTCD14 year& operator+=(const years& y) NOEXCEPT;
+    CONSTCD14 year& operator-=(const years& y) NOEXCEPT;
+
+    CONSTCD11 year operator-() const NOEXCEPT;
+    CONSTCD11 year operator+() const NOEXCEPT;
+
+    CONSTCD11 bool is_leap() const NOEXCEPT;
+
+    CONSTCD11 explicit operator int() const NOEXCEPT;
+    CONSTCD11 bool ok() const NOEXCEPT;
+
+    static CONSTCD11 year min() NOEXCEPT { return year{-32767}; }
+    static CONSTCD11 year max() NOEXCEPT { return year{32767}; }
+};
+
+CONSTCD11 bool operator==(const year& x, const year& y) NOEXCEPT;
+CONSTCD11 bool operator!=(const year& x, const year& y) NOEXCEPT;
+CONSTCD11 bool operator< (const year& x, const year& y) NOEXCEPT;
+CONSTCD11 bool operator> (const year& x, const year& y) NOEXCEPT;
+CONSTCD11 bool operator<=(const year& x, const year& y) NOEXCEPT;
+CONSTCD11 bool operator>=(const year& x, const year& y) NOEXCEPT;
+
+CONSTCD11 year  operator+(const year&  x, const years& y) NOEXCEPT;
+CONSTCD11 year  operator+(const years& x, const year&  y) NOEXCEPT;
+CONSTCD11 year  operator-(const year&  x, const years& y) NOEXCEPT;
+CONSTCD11 years operator-(const year&  x, const year&  y) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year& y);
+
+// weekday
+
+class weekday
+{
+    unsigned char wd_;
+public:
+    weekday() = default;
+    explicit CONSTCD11 weekday(unsigned wd) NOEXCEPT;
+    CONSTCD14 weekday(const sys_days& dp) NOEXCEPT;
+    CONSTCD14 explicit weekday(const local_days& dp) NOEXCEPT;
+
+    CONSTCD14 weekday& operator++()    NOEXCEPT;
+    CONSTCD14 weekday  operator++(int) NOEXCEPT;
+    CONSTCD14 weekday& operator--()    NOEXCEPT;
+    CONSTCD14 weekday  operator--(int) NOEXCEPT;
+
+    CONSTCD14 weekday& operator+=(const days& d) NOEXCEPT;
+    CONSTCD14 weekday& operator-=(const days& d) NOEXCEPT;
+
+    CONSTCD11 bool ok() const NOEXCEPT;
+
+    CONSTCD11 unsigned c_encoding() const NOEXCEPT;
+    CONSTCD11 unsigned iso_encoding() const NOEXCEPT;
+
+    CONSTCD11 weekday_indexed operator[](unsigned index) const NOEXCEPT;
+    CONSTCD11 weekday_last    operator[](last_spec)      const NOEXCEPT;
+
+private:
+    static CONSTCD14 unsigned char weekday_from_days(int z) NOEXCEPT;
+
+    friend CONSTCD11 bool operator==(const weekday& x, const weekday& y) NOEXCEPT;
+    friend CONSTCD14 days operator-(const weekday& x, const weekday& y) NOEXCEPT;
+    friend CONSTCD14 weekday operator+(const weekday& x, const days& y) NOEXCEPT;
+    template<class CharT, class Traits>
+        friend std::basic_ostream<CharT, Traits>&
+            operator<<(std::basic_ostream<CharT, Traits>& os, const weekday& wd);
+    friend class weekday_indexed;
+};
+
+CONSTCD11 bool operator==(const weekday& x, const weekday& y) NOEXCEPT;
+CONSTCD11 bool operator!=(const weekday& x, const weekday& y) NOEXCEPT;
+
+CONSTCD14 weekday operator+(const weekday& x, const days&    y) NOEXCEPT;
+CONSTCD14 weekday operator+(const days&    x, const weekday& y) NOEXCEPT;
+CONSTCD14 weekday operator-(const weekday& x, const days&    y) NOEXCEPT;
+CONSTCD14 days    operator-(const weekday& x, const weekday& y) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const weekday& wd);
+
+// weekday_indexed
+
+class weekday_indexed
+{
+    unsigned char wd_    : 4;
+    unsigned char index_ : 4;
+
+public:
+    weekday_indexed() = default;
+    CONSTCD11 weekday_indexed(const date::weekday& wd, unsigned index) NOEXCEPT;
+
+    CONSTCD11 date::weekday weekday() const NOEXCEPT;
+    CONSTCD11 unsigned index() const NOEXCEPT;
+    CONSTCD11 bool ok() const NOEXCEPT;
+};
+
+CONSTCD11 bool operator==(const weekday_indexed& x, const weekday_indexed& y) NOEXCEPT;
+CONSTCD11 bool operator!=(const weekday_indexed& x, const weekday_indexed& y) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const weekday_indexed& wdi);
+
+// weekday_last
+
+class weekday_last
+{
+    date::weekday wd_;
+
+public:
+    explicit CONSTCD11 weekday_last(const date::weekday& wd) NOEXCEPT;
+
+    CONSTCD11 date::weekday weekday() const NOEXCEPT;
+    CONSTCD11 bool ok() const NOEXCEPT;
+};
+
+CONSTCD11 bool operator==(const weekday_last& x, const weekday_last& y) NOEXCEPT;
+CONSTCD11 bool operator!=(const weekday_last& x, const weekday_last& y) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const weekday_last& wdl);
+
+namespace detail
+{
+
+struct unspecified_month_disambiguator {};
+
+}  // namespace detail
+
+// year_month
+
+class year_month
+{
+    date::year  y_;
+    date::month m_;
+
+public:
+    year_month() = default;
+    CONSTCD11 year_month(const date::year& y, const date::month& m) NOEXCEPT;
+
+    CONSTCD11 date::year  year()  const NOEXCEPT;
+    CONSTCD11 date::month month() const NOEXCEPT;
+
+    template<class = detail::unspecified_month_disambiguator>
+    CONSTCD14 year_month& operator+=(const months& dm) NOEXCEPT;
+    template<class = detail::unspecified_month_disambiguator>
+    CONSTCD14 year_month& operator-=(const months& dm) NOEXCEPT;
+    CONSTCD14 year_month& operator+=(const years& dy) NOEXCEPT;
+    CONSTCD14 year_month& operator-=(const years& dy) NOEXCEPT;
+
+    CONSTCD11 bool ok() const NOEXCEPT;
+};
+
+CONSTCD11 bool operator==(const year_month& x, const year_month& y) NOEXCEPT;
+CONSTCD11 bool operator!=(const year_month& x, const year_month& y) NOEXCEPT;
+CONSTCD11 bool operator< (const year_month& x, const year_month& y) NOEXCEPT;
+CONSTCD11 bool operator> (const year_month& x, const year_month& y) NOEXCEPT;
+CONSTCD11 bool operator<=(const year_month& x, const year_month& y) NOEXCEPT;
+CONSTCD11 bool operator>=(const year_month& x, const year_month& y) NOEXCEPT;
+
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14 year_month operator+(const year_month& ym, const months& dm) NOEXCEPT;
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14 year_month operator+(const months& dm, const year_month& ym) NOEXCEPT;
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14 year_month operator-(const year_month& ym, const months& dm) NOEXCEPT;
+
+CONSTCD11 months operator-(const year_month& x, const year_month& y) NOEXCEPT;
+CONSTCD11 year_month operator+(const year_month& ym, const years& dy) NOEXCEPT;
+CONSTCD11 year_month operator+(const years& dy, const year_month& ym) NOEXCEPT;
+CONSTCD11 year_month operator-(const year_month& ym, const years& dy) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year_month& ym);
+
+// month_day
+
+class month_day
+{
+    date::month m_;
+    date::day   d_;
+
+public:
+    month_day() = default;
+    CONSTCD11 month_day(const date::month& m, const date::day& d) NOEXCEPT;
+
+    CONSTCD11 date::month month() const NOEXCEPT;
+    CONSTCD11 date::day   day() const NOEXCEPT;
+
+    CONSTCD14 bool ok() const NOEXCEPT;
+};
+
+CONSTCD11 bool operator==(const month_day& x, const month_day& y) NOEXCEPT;
+CONSTCD11 bool operator!=(const month_day& x, const month_day& y) NOEXCEPT;
+CONSTCD11 bool operator< (const month_day& x, const month_day& y) NOEXCEPT;
+CONSTCD11 bool operator> (const month_day& x, const month_day& y) NOEXCEPT;
+CONSTCD11 bool operator<=(const month_day& x, const month_day& y) NOEXCEPT;
+CONSTCD11 bool operator>=(const month_day& x, const month_day& y) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const month_day& md);
+
+// month_day_last
+
+class month_day_last
+{
+    date::month m_;
+
+public:
+    CONSTCD11 explicit month_day_last(const date::month& m) NOEXCEPT;
+
+    CONSTCD11 date::month month() const NOEXCEPT;
+    CONSTCD11 bool ok() const NOEXCEPT;
+};
+
+CONSTCD11 bool operator==(const month_day_last& x, const month_day_last& y) NOEXCEPT;
+CONSTCD11 bool operator!=(const month_day_last& x, const month_day_last& y) NOEXCEPT;
+CONSTCD11 bool operator< (const month_day_last& x, const month_day_last& y) NOEXCEPT;
+CONSTCD11 bool operator> (const month_day_last& x, const month_day_last& y) NOEXCEPT;
+CONSTCD11 bool operator<=(const month_day_last& x, const month_day_last& y) NOEXCEPT;
+CONSTCD11 bool operator>=(const month_day_last& x, const month_day_last& y) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const month_day_last& mdl);
+
+// month_weekday
+
+class month_weekday
+{
+    date::month           m_;
+    date::weekday_indexed wdi_;
+public:
+    CONSTCD11 month_weekday(const date::month& m,
+                            const date::weekday_indexed& wdi) NOEXCEPT;
+
+    CONSTCD11 date::month           month()           const NOEXCEPT;
+    CONSTCD11 date::weekday_indexed weekday_indexed() const NOEXCEPT;
+
+    CONSTCD11 bool ok() const NOEXCEPT;
+};
+
+CONSTCD11 bool operator==(const month_weekday& x, const month_weekday& y) NOEXCEPT;
+CONSTCD11 bool operator!=(const month_weekday& x, const month_weekday& y) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const month_weekday& mwd);
+
+// month_weekday_last
+
+class month_weekday_last
+{
+    date::month        m_;
+    date::weekday_last wdl_;
+
+public:
+    CONSTCD11 month_weekday_last(const date::month& m,
+                                 const date::weekday_last& wd) NOEXCEPT;
+
+    CONSTCD11 date::month        month()        const NOEXCEPT;
+    CONSTCD11 date::weekday_last weekday_last() const NOEXCEPT;
+
+    CONSTCD11 bool ok() const NOEXCEPT;
+};
+
+CONSTCD11
+    bool operator==(const month_weekday_last& x, const month_weekday_last& y) NOEXCEPT;
+CONSTCD11
+    bool operator!=(const month_weekday_last& x, const month_weekday_last& y) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const month_weekday_last& mwdl);
+
+// class year_month_day
+
+class year_month_day
+{
+    date::year  y_;
+    date::month m_;
+    date::day   d_;
+
+public:
+    year_month_day() = default;
+    CONSTCD11 year_month_day(const date::year& y, const date::month& m,
+                             const date::day& d) NOEXCEPT;
+    CONSTCD14 year_month_day(const year_month_day_last& ymdl) NOEXCEPT;
+
+    CONSTCD14 year_month_day(sys_days dp) NOEXCEPT;
+    CONSTCD14 explicit year_month_day(local_days dp) NOEXCEPT;
+
+    template<class = detail::unspecified_month_disambiguator>
+    CONSTCD14 year_month_day& operator+=(const months& m) NOEXCEPT;
+    template<class = detail::unspecified_month_disambiguator>
+    CONSTCD14 year_month_day& operator-=(const months& m) NOEXCEPT;
+    CONSTCD14 year_month_day& operator+=(const years& y)  NOEXCEPT;
+    CONSTCD14 year_month_day& operator-=(const years& y)  NOEXCEPT;
+
+    CONSTCD11 date::year  year()  const NOEXCEPT;
+    CONSTCD11 date::month month() const NOEXCEPT;
+    CONSTCD11 date::day   day()   const NOEXCEPT;
+
+    CONSTCD14 operator sys_days() const NOEXCEPT;
+    CONSTCD14 explicit operator local_days() const NOEXCEPT;
+    CONSTCD14 bool ok() const NOEXCEPT;
+
+private:
+    static CONSTCD14 year_month_day from_days(days dp) NOEXCEPT;
+    CONSTCD14 days to_days() const NOEXCEPT;
+};
+
+CONSTCD11 bool operator==(const year_month_day& x, const year_month_day& y) NOEXCEPT;
+CONSTCD11 bool operator!=(const year_month_day& x, const year_month_day& y) NOEXCEPT;
+CONSTCD11 bool operator< (const year_month_day& x, const year_month_day& y) NOEXCEPT;
+CONSTCD11 bool operator> (const year_month_day& x, const year_month_day& y) NOEXCEPT;
+CONSTCD11 bool operator<=(const year_month_day& x, const year_month_day& y) NOEXCEPT;
+CONSTCD11 bool operator>=(const year_month_day& x, const year_month_day& y) NOEXCEPT;
+
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14 year_month_day operator+(const year_month_day& ymd, const months& dm) NOEXCEPT;
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14 year_month_day operator+(const months& dm, const year_month_day& ymd) NOEXCEPT;
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14 year_month_day operator-(const year_month_day& ymd, const months& dm) NOEXCEPT;
+CONSTCD11 year_month_day operator+(const year_month_day& ymd, const years& dy)  NOEXCEPT;
+CONSTCD11 year_month_day operator+(const years& dy, const year_month_day& ymd)  NOEXCEPT;
+CONSTCD11 year_month_day operator-(const year_month_day& ymd, const years& dy)  NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_day& ymd);
+
+// year_month_day_last
+
+class year_month_day_last
+{
+    date::year           y_;
+    date::month_day_last mdl_;
+
+public:
+    CONSTCD11 year_month_day_last(const date::year& y,
+                                  const date::month_day_last& mdl) NOEXCEPT;
+
+    template<class = detail::unspecified_month_disambiguator>
+    CONSTCD14 year_month_day_last& operator+=(const months& m) NOEXCEPT;
+    template<class = detail::unspecified_month_disambiguator>
+    CONSTCD14 year_month_day_last& operator-=(const months& m) NOEXCEPT;
+    CONSTCD14 year_month_day_last& operator+=(const years& y)  NOEXCEPT;
+    CONSTCD14 year_month_day_last& operator-=(const years& y)  NOEXCEPT;
+
+    CONSTCD11 date::year           year()           const NOEXCEPT;
+    CONSTCD11 date::month          month()          const NOEXCEPT;
+    CONSTCD11 date::month_day_last month_day_last() const NOEXCEPT;
+    CONSTCD14 date::day            day()            const NOEXCEPT;
+
+    CONSTCD14 operator sys_days() const NOEXCEPT;
+    CONSTCD14 explicit operator local_days() const NOEXCEPT;
+    CONSTCD11 bool ok() const NOEXCEPT;
+};
+
+CONSTCD11
+    bool operator==(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT;
+CONSTCD11
+    bool operator!=(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT;
+CONSTCD11
+    bool operator< (const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT;
+CONSTCD11
+    bool operator> (const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT;
+CONSTCD11
+    bool operator<=(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT;
+CONSTCD11
+    bool operator>=(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT;
+
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14
+year_month_day_last
+operator+(const year_month_day_last& ymdl, const months& dm) NOEXCEPT;
+
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14
+year_month_day_last
+operator+(const months& dm, const year_month_day_last& ymdl) NOEXCEPT;
+
+CONSTCD11
+year_month_day_last
+operator+(const year_month_day_last& ymdl, const years& dy) NOEXCEPT;
+
+CONSTCD11
+year_month_day_last
+operator+(const years& dy, const year_month_day_last& ymdl) NOEXCEPT;
+
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14
+year_month_day_last
+operator-(const year_month_day_last& ymdl, const months& dm) NOEXCEPT;
+
+CONSTCD11
+year_month_day_last
+operator-(const year_month_day_last& ymdl, const years& dy) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_day_last& ymdl);
+
+// year_month_weekday
+
+class year_month_weekday
+{
+    date::year            y_;
+    date::month           m_;
+    date::weekday_indexed wdi_;
+
+public:
+    year_month_weekday() = default;
+    CONSTCD11 year_month_weekday(const date::year& y, const date::month& m,
+                                   const date::weekday_indexed& wdi) NOEXCEPT;
+    CONSTCD14 year_month_weekday(const sys_days& dp) NOEXCEPT;
+    CONSTCD14 explicit year_month_weekday(const local_days& dp) NOEXCEPT;
+
+    template<class = detail::unspecified_month_disambiguator>
+    CONSTCD14 year_month_weekday& operator+=(const months& m) NOEXCEPT;
+    template<class = detail::unspecified_month_disambiguator>
+    CONSTCD14 year_month_weekday& operator-=(const months& m) NOEXCEPT;
+    CONSTCD14 year_month_weekday& operator+=(const years& y)  NOEXCEPT;
+    CONSTCD14 year_month_weekday& operator-=(const years& y)  NOEXCEPT;
+
+    CONSTCD11 date::year year() const NOEXCEPT;
+    CONSTCD11 date::month month() const NOEXCEPT;
+    CONSTCD11 date::weekday weekday() const NOEXCEPT;
+    CONSTCD11 unsigned index() const NOEXCEPT;
+    CONSTCD11 date::weekday_indexed weekday_indexed() const NOEXCEPT;
+
+    CONSTCD14 operator sys_days() const NOEXCEPT;
+    CONSTCD14 explicit operator local_days() const NOEXCEPT;
+    CONSTCD14 bool ok() const NOEXCEPT;
+
+private:
+    static CONSTCD14 year_month_weekday from_days(days dp) NOEXCEPT;
+    CONSTCD14 days to_days() const NOEXCEPT;
+};
+
+CONSTCD11
+    bool operator==(const year_month_weekday& x, const year_month_weekday& y) NOEXCEPT;
+CONSTCD11
+    bool operator!=(const year_month_weekday& x, const year_month_weekday& y) NOEXCEPT;
+
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14
+year_month_weekday
+operator+(const year_month_weekday& ymwd, const months& dm) NOEXCEPT;
+
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14
+year_month_weekday
+operator+(const months& dm, const year_month_weekday& ymwd) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday
+operator+(const year_month_weekday& ymwd, const years& dy) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday
+operator+(const years& dy, const year_month_weekday& ymwd) NOEXCEPT;
+
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14
+year_month_weekday
+operator-(const year_month_weekday& ymwd, const months& dm) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday
+operator-(const year_month_weekday& ymwd, const years& dy) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_weekday& ymwdi);
+
+// year_month_weekday_last
+
+class year_month_weekday_last
+{
+    date::year y_;
+    date::month m_;
+    date::weekday_last wdl_;
+
+public:
+    CONSTCD11 year_month_weekday_last(const date::year& y, const date::month& m,
+                                      const date::weekday_last& wdl) NOEXCEPT;
+
+    template<class = detail::unspecified_month_disambiguator>
+    CONSTCD14 year_month_weekday_last& operator+=(const months& m) NOEXCEPT;
+    template<class = detail::unspecified_month_disambiguator>
+    CONSTCD14 year_month_weekday_last& operator-=(const months& m) NOEXCEPT;
+    CONSTCD14 year_month_weekday_last& operator+=(const years& y) NOEXCEPT;
+    CONSTCD14 year_month_weekday_last& operator-=(const years& y) NOEXCEPT;
+
+    CONSTCD11 date::year year() const NOEXCEPT;
+    CONSTCD11 date::month month() const NOEXCEPT;
+    CONSTCD11 date::weekday weekday() const NOEXCEPT;
+    CONSTCD11 date::weekday_last weekday_last() const NOEXCEPT;
+
+    CONSTCD14 operator sys_days() const NOEXCEPT;
+    CONSTCD14 explicit operator local_days() const NOEXCEPT;
+    CONSTCD11 bool ok() const NOEXCEPT;
+
+private:
+    CONSTCD14 days to_days() const NOEXCEPT;
+};
+
+CONSTCD11
+bool
+operator==(const year_month_weekday_last& x, const year_month_weekday_last& y) NOEXCEPT;
+
+CONSTCD11
+bool
+operator!=(const year_month_weekday_last& x, const year_month_weekday_last& y) NOEXCEPT;
+
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14
+year_month_weekday_last
+operator+(const year_month_weekday_last& ymwdl, const months& dm) NOEXCEPT;
+
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14
+year_month_weekday_last
+operator+(const months& dm, const year_month_weekday_last& ymwdl) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday_last
+operator+(const year_month_weekday_last& ymwdl, const years& dy) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday_last
+operator+(const years& dy, const year_month_weekday_last& ymwdl) NOEXCEPT;
+
+template<class = detail::unspecified_month_disambiguator>
+CONSTCD14
+year_month_weekday_last
+operator-(const year_month_weekday_last& ymwdl, const months& dm) NOEXCEPT;
+
+CONSTCD11
+year_month_weekday_last
+operator-(const year_month_weekday_last& ymwdl, const years& dy) NOEXCEPT;
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_weekday_last& ymwdl);
+
+#if !defined(_MSC_VER) || (_MSC_VER >= 1900)
+inline namespace literals
+{
+
+CONSTCD11 date::day  operator "" _d(unsigned long long d) NOEXCEPT;
+CONSTCD11 date::year operator "" _y(unsigned long long y) NOEXCEPT;
+
+}  // inline namespace literals
+#endif // !defined(_MSC_VER) || (_MSC_VER >= 1900)
+
+// CONSTDATA date::month January{1};
+// CONSTDATA date::month February{2};
+// CONSTDATA date::month March{3};
+// CONSTDATA date::month April{4};
+// CONSTDATA date::month May{5};
+// CONSTDATA date::month June{6};
+// CONSTDATA date::month July{7};
+// CONSTDATA date::month August{8};
+// CONSTDATA date::month September{9};
+// CONSTDATA date::month October{10};
+// CONSTDATA date::month November{11};
+// CONSTDATA date::month December{12};
+//
+// CONSTDATA date::weekday Sunday{0u};
+// CONSTDATA date::weekday Monday{1u};
+// CONSTDATA date::weekday Tuesday{2u};
+// CONSTDATA date::weekday Wednesday{3u};
+// CONSTDATA date::weekday Thursday{4u};
+// CONSTDATA date::weekday Friday{5u};
+// CONSTDATA date::weekday Saturday{6u};
+
+#if HAS_VOID_T
+
+template <class T, class = std::void_t<>>
+struct is_clock
+    : std::false_type
+{};
+
+template <class T>
+struct is_clock<T, std::void_t<decltype(T::now()), typename T::rep, typename T::period,
+                               typename T::duration, typename T::time_point,
+                               decltype(T::is_steady)>>
+    : std::true_type
+{};
+
+template<class T> inline constexpr bool is_clock_v = is_clock<T>::value;
+
+#endif  // HAS_VOID_T
+
+//----------------+
+// Implementation |
+//----------------+
+
+// utilities
+namespace detail {
+
+template<class CharT, class Traits = std::char_traits<CharT>>
+class save_istream
+{
+protected:
+    std::basic_ios<CharT, Traits>& is_;
+    CharT fill_;
+    std::ios::fmtflags flags_;
+    std::streamsize precision_;
+    std::streamsize width_;
+    std::basic_ostream<CharT, Traits>* tie_;
+    std::locale loc_;
+
+public:
+    ~save_istream()
+    {
+        is_.fill(fill_);
+        is_.flags(flags_);
+        is_.precision(precision_);
+        is_.width(width_);
+        is_.imbue(loc_);
+        is_.tie(tie_);
+    }
+
+    save_istream(const save_istream&) = delete;
+    save_istream& operator=(const save_istream&) = delete;
+
+    explicit save_istream(std::basic_ios<CharT, Traits>& is)
+        : is_(is)
+        , fill_(is.fill())
+        , flags_(is.flags())
+        , precision_(is.precision())
+        , width_(is.width(0))
+        , tie_(is.tie(nullptr))
+        , loc_(is.getloc())
+        {
+            if (tie_ != nullptr)
+                tie_->flush();
+        }
+};
+
+template<class CharT, class Traits = std::char_traits<CharT>>
+class save_ostream
+    : private save_istream<CharT, Traits>
+{
+public:
+    ~save_ostream()
+    {
+        if ((this->flags_ & std::ios::unitbuf) &&
+#if HAS_UNCAUGHT_EXCEPTIONS
+                std::uncaught_exceptions() == 0 &&
+#else
+                !std::uncaught_exception() &&
+#endif
+                this->is_.good())
+            this->is_.rdbuf()->pubsync();
+    }
+
+    save_ostream(const save_ostream&) = delete;
+    save_ostream& operator=(const save_ostream&) = delete;
+
+    explicit save_ostream(std::basic_ios<CharT, Traits>& os)
+        : save_istream<CharT, Traits>(os)
+        {
+        }
+};
+
+template <class T>
+struct choose_trunc_type
+{
+    static const int digits = std::numeric_limits<T>::digits;
+    using type = typename std::conditional
+                 <
+                     digits < 32,
+                     std::int32_t,
+                     typename std::conditional
+                     <
+                         digits < 64,
+                         std::int64_t,
+#ifdef __SIZEOF_INT128__
+                         __int128
+#else
+                         std::int64_t
+#endif
+                     >::type
+                 >::type;
+};
+
+template <class T>
+CONSTCD11
+inline
+typename std::enable_if
+<
+    !std::chrono::treat_as_floating_point<T>::value,
+    T
+>::type
+trunc(T t) NOEXCEPT
+{
+    return t;
+}
+
+template <class T>
+CONSTCD14
+inline
+typename std::enable_if
+<
+    std::chrono::treat_as_floating_point<T>::value,
+    T
+>::type
+trunc(T t) NOEXCEPT
+{
+    using std::numeric_limits;
+    using I = typename choose_trunc_type<T>::type;
+    CONSTDATA auto digits = numeric_limits<T>::digits;
+    static_assert(digits < numeric_limits<I>::digits, "");
+    CONSTDATA auto max = I{1} << (digits-1);
+    CONSTDATA auto min = -max;
+    const auto negative = t < T{0};
+    if (min <= t && t <= max && t != 0 && t == t)
+    {
+        t = static_cast<T>(static_cast<I>(t));
+        if (t == 0 && negative)
+            t = -t;
+    }
+    return t;
+}
+
+template <std::intmax_t Xp, std::intmax_t Yp>
+struct static_gcd
+{
+    static const std::intmax_t value = static_gcd<Yp, Xp % Yp>::value;
+};
+
+template <std::intmax_t Xp>
+struct static_gcd<Xp, 0>
+{
+    static const std::intmax_t value = Xp;
+};
+
+template <>
+struct static_gcd<0, 0>
+{
+    static const std::intmax_t value = 1;
+};
+
+template <class R1, class R2>
+struct no_overflow
+{
+private:
+    static const std::intmax_t gcd_n1_n2 = static_gcd<R1::num, R2::num>::value;
+    static const std::intmax_t gcd_d1_d2 = static_gcd<R1::den, R2::den>::value;
+    static const std::intmax_t n1 = R1::num / gcd_n1_n2;
+    static const std::intmax_t d1 = R1::den / gcd_d1_d2;
+    static const std::intmax_t n2 = R2::num / gcd_n1_n2;
+    static const std::intmax_t d2 = R2::den / gcd_d1_d2;
+#ifdef __cpp_constexpr
+    static const std::intmax_t max = std::numeric_limits<std::intmax_t>::max();
+#else
+    static const std::intmax_t max = LLONG_MAX;
+#endif
+
+    template <std::intmax_t Xp, std::intmax_t Yp, bool overflow>
+    struct mul    // overflow == false
+    {
+        static const std::intmax_t value = Xp * Yp;
+    };
+
+    template <std::intmax_t Xp, std::intmax_t Yp>
+    struct mul<Xp, Yp, true>
+    {
+        static const std::intmax_t value = 1;
+    };
+
+public:
+    static const bool value = (n1 <= max / d2) && (n2 <= max / d1);
+    typedef std::ratio<mul<n1, d2, !value>::value,
+                       mul<n2, d1, !value>::value> type;
+};
+
+}  // detail
+
+// trunc towards zero
+template <class To, class Rep, class Period>
+CONSTCD11
+inline
+typename std::enable_if
+<
+    detail::no_overflow<Period, typename To::period>::value,
+    To
+>::type
+trunc(const std::chrono::duration<Rep, Period>& d)
+{
+    return To{detail::trunc(std::chrono::duration_cast<To>(d).count())};
+}
+
+template <class To, class Rep, class Period>
+CONSTCD11
+inline
+typename std::enable_if
+<
+    !detail::no_overflow<Period, typename To::period>::value,
+    To
+>::type
+trunc(const std::chrono::duration<Rep, Period>& d)
+{
+    using std::chrono::duration_cast;
+    using std::chrono::duration;
+    using rep = typename std::common_type<Rep, typename To::rep>::type;
+    return To{detail::trunc(duration_cast<To>(duration_cast<duration<rep>>(d)).count())};
+}
+
+#ifndef HAS_CHRONO_ROUNDING
+#  if defined(_MSC_FULL_VER) && (_MSC_FULL_VER >= 190023918 || (_MSC_FULL_VER >= 190000000 && defined (__clang__)))
+#    define HAS_CHRONO_ROUNDING 1
+#  elif defined(__cpp_lib_chrono) && __cplusplus > 201402 && __cpp_lib_chrono >= 201510
+#    define HAS_CHRONO_ROUNDING 1
+#  elif defined(_LIBCPP_VERSION) && __cplusplus > 201402 && _LIBCPP_VERSION >= 3800
+#    define HAS_CHRONO_ROUNDING 1
+#  else
+#    define HAS_CHRONO_ROUNDING 0
+#  endif
+#endif  // HAS_CHRONO_ROUNDING
+
+#if HAS_CHRONO_ROUNDING == 0
+
+// round down
+template <class To, class Rep, class Period>
+CONSTCD14
+inline
+typename std::enable_if
+<
+    detail::no_overflow<Period, typename To::period>::value,
+    To
+>::type
+floor(const std::chrono::duration<Rep, Period>& d)
+{
+    auto t = trunc<To>(d);
+    if (t > d)
+        return t - To{1};
+    return t;
+}
+
+template <class To, class Rep, class Period>
+CONSTCD14
+inline
+typename std::enable_if
+<
+    !detail::no_overflow<Period, typename To::period>::value,
+    To
+>::type
+floor(const std::chrono::duration<Rep, Period>& d)
+{
+    using rep = typename std::common_type<Rep, typename To::rep>::type;
+    return floor<To>(floor<std::chrono::duration<rep>>(d));
+}
+
+// round to nearest, to even on tie
+template <class To, class Rep, class Period>
+CONSTCD14
+inline
+To
+round(const std::chrono::duration<Rep, Period>& d)
+{
+    auto t0 = floor<To>(d);
+    auto t1 = t0 + To{1};
+    if (t1 == To{0} && t0 < To{0})
+        t1 = -t1;
+    auto diff0 = d - t0;
+    auto diff1 = t1 - d;
+    if (diff0 == diff1)
+    {
+        if (t0 - trunc<To>(t0/2)*2 == To{0})
+            return t0;
+        return t1;
+    }
+    if (diff0 < diff1)
+        return t0;
+    return t1;
+}
+
+// round up
+template <class To, class Rep, class Period>
+CONSTCD14
+inline
+To
+ceil(const std::chrono::duration<Rep, Period>& d)
+{
+    auto t = trunc<To>(d);
+    if (t < d)
+        return t + To{1};
+    return t;
+}
+
+template <class Rep, class Period,
+          class = typename std::enable_if
+          <
+              std::numeric_limits<Rep>::is_signed
+          >::type>
+CONSTCD11
+std::chrono::duration<Rep, Period>
+abs(std::chrono::duration<Rep, Period> d)
+{
+    return d >= d.zero() ? d : static_cast<decltype(d)>(-d);
+}
+
+// round down
+template <class To, class Clock, class FromDuration>
+CONSTCD11
+inline
+std::chrono::time_point<Clock, To>
+floor(const std::chrono::time_point<Clock, FromDuration>& tp)
+{
+    using std::chrono::time_point;
+    return time_point<Clock, To>{date::floor<To>(tp.time_since_epoch())};
+}
+
+// round to nearest, to even on tie
+template <class To, class Clock, class FromDuration>
+CONSTCD11
+inline
+std::chrono::time_point<Clock, To>
+round(const std::chrono::time_point<Clock, FromDuration>& tp)
+{
+    using std::chrono::time_point;
+    return time_point<Clock, To>{round<To>(tp.time_since_epoch())};
+}
+
+// round up
+template <class To, class Clock, class FromDuration>
+CONSTCD11
+inline
+std::chrono::time_point<Clock, To>
+ceil(const std::chrono::time_point<Clock, FromDuration>& tp)
+{
+    using std::chrono::time_point;
+    return time_point<Clock, To>{ceil<To>(tp.time_since_epoch())};
+}
+
+#else  // HAS_CHRONO_ROUNDING == 1
+
+using std::chrono::floor;
+using std::chrono::ceil;
+using std::chrono::round;
+using std::chrono::abs;
+
+#endif  // HAS_CHRONO_ROUNDING
+
+namespace detail
+{
+
+template <class To, class Rep, class Period>
+CONSTCD14
+inline
+typename std::enable_if
+<
+    !std::chrono::treat_as_floating_point<typename To::rep>::value,
+    To
+>::type
+round_i(const std::chrono::duration<Rep, Period>& d)
+{
+    return round<To>(d);
+}
+
+template <class To, class Rep, class Period>
+CONSTCD14
+inline
+typename std::enable_if
+<
+    std::chrono::treat_as_floating_point<typename To::rep>::value,
+    To
+>::type
+round_i(const std::chrono::duration<Rep, Period>& d)
+{
+    return d;
+}
+
+template <class To, class Clock, class FromDuration>
+CONSTCD11
+inline
+std::chrono::time_point<Clock, To>
+round_i(const std::chrono::time_point<Clock, FromDuration>& tp)
+{
+    using std::chrono::time_point;
+    return time_point<Clock, To>{round_i<To>(tp.time_since_epoch())};
+}
+
+}  // detail
+
+// trunc towards zero
+template <class To, class Clock, class FromDuration>
+CONSTCD11
+inline
+std::chrono::time_point<Clock, To>
+trunc(const std::chrono::time_point<Clock, FromDuration>& tp)
+{
+    using std::chrono::time_point;
+    return time_point<Clock, To>{trunc<To>(tp.time_since_epoch())};
+}
+
+// day
+
+CONSTCD11 inline day::day(unsigned d) NOEXCEPT : d_(static_cast<decltype(d_)>(d)) {}
+CONSTCD14 inline day& day::operator++() NOEXCEPT {++d_; return *this;}
+CONSTCD14 inline day day::operator++(int) NOEXCEPT {auto tmp(*this); ++(*this); return tmp;}
+CONSTCD14 inline day& day::operator--() NOEXCEPT {--d_; return *this;}
+CONSTCD14 inline day day::operator--(int) NOEXCEPT {auto tmp(*this); --(*this); return tmp;}
+CONSTCD14 inline day& day::operator+=(const days& d) NOEXCEPT {*this = *this + d; return *this;}
+CONSTCD14 inline day& day::operator-=(const days& d) NOEXCEPT {*this = *this - d; return *this;}
+CONSTCD11 inline day::operator unsigned() const NOEXCEPT {return d_;}
+CONSTCD11 inline bool day::ok() const NOEXCEPT {return 1 <= d_ && d_ <= 31;}
+
+CONSTCD11
+inline
+bool
+operator==(const day& x, const day& y) NOEXCEPT
+{
+    return static_cast<unsigned>(x) == static_cast<unsigned>(y);
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const day& x, const day& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+CONSTCD11
+inline
+bool
+operator<(const day& x, const day& y) NOEXCEPT
+{
+    return static_cast<unsigned>(x) < static_cast<unsigned>(y);
+}
+
+CONSTCD11
+inline
+bool
+operator>(const day& x, const day& y) NOEXCEPT
+{
+    return y < x;
+}
+
+CONSTCD11
+inline
+bool
+operator<=(const day& x, const day& y) NOEXCEPT
+{
+    return !(y < x);
+}
+
+CONSTCD11
+inline
+bool
+operator>=(const day& x, const day& y) NOEXCEPT
+{
+    return !(x < y);
+}
+
+CONSTCD11
+inline
+days
+operator-(const day& x, const day& y) NOEXCEPT
+{
+    return days{static_cast<days::rep>(static_cast<unsigned>(x)
+                                     - static_cast<unsigned>(y))};
+}
+
+CONSTCD11
+inline
+day
+operator+(const day& x, const days& y) NOEXCEPT
+{
+    return day{static_cast<unsigned>(x) + static_cast<unsigned>(y.count())};
+}
+
+CONSTCD11
+inline
+day
+operator+(const days& x, const day& y) NOEXCEPT
+{
+    return y + x;
+}
+
+CONSTCD11
+inline
+day
+operator-(const day& x, const days& y) NOEXCEPT
+{
+    return x + -y;
+}
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const day& d)
+{
+    detail::save_ostream<CharT, Traits> _(os);
+    os.fill('0');
+    os.flags(std::ios::dec | std::ios::right);
+    os.width(2);
+    os << static_cast<unsigned>(d);
+    return os;
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const day& d)
+{
+    detail::low_level_fmt(os, d);
+    if (!d.ok())
+        os << " is not a valid day";
+    return os;
+}
+
+// month
+
+CONSTCD11 inline month::month(unsigned m) NOEXCEPT : m_(static_cast<decltype(m_)>(m)) {}
+CONSTCD14 inline month& month::operator++() NOEXCEPT {*this += months{1}; return *this;}
+CONSTCD14 inline month month::operator++(int) NOEXCEPT {auto tmp(*this); ++(*this); return tmp;}
+CONSTCD14 inline month& month::operator--() NOEXCEPT {*this -= months{1}; return *this;}
+CONSTCD14 inline month month::operator--(int) NOEXCEPT {auto tmp(*this); --(*this); return tmp;}
+
+CONSTCD14
+inline
+month&
+month::operator+=(const months& m) NOEXCEPT
+{
+    *this = *this + m;
+    return *this;
+}
+
+CONSTCD14
+inline
+month&
+month::operator-=(const months& m) NOEXCEPT
+{
+    *this = *this - m;
+    return *this;
+}
+
+CONSTCD11 inline month::operator unsigned() const NOEXCEPT {return m_;}
+CONSTCD11 inline bool month::ok() const NOEXCEPT {return 1 <= m_ && m_ <= 12;}
+
+CONSTCD11
+inline
+bool
+operator==(const month& x, const month& y) NOEXCEPT
+{
+    return static_cast<unsigned>(x) == static_cast<unsigned>(y);
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const month& x, const month& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+CONSTCD11
+inline
+bool
+operator<(const month& x, const month& y) NOEXCEPT
+{
+    return static_cast<unsigned>(x) < static_cast<unsigned>(y);
+}
+
+CONSTCD11
+inline
+bool
+operator>(const month& x, const month& y) NOEXCEPT
+{
+    return y < x;
+}
+
+CONSTCD11
+inline
+bool
+operator<=(const month& x, const month& y) NOEXCEPT
+{
+    return !(y < x);
+}
+
+CONSTCD11
+inline
+bool
+operator>=(const month& x, const month& y) NOEXCEPT
+{
+    return !(x < y);
+}
+
+CONSTCD14
+inline
+months
+operator-(const month& x, const month& y) NOEXCEPT
+{
+    auto const d = static_cast<unsigned>(x) - static_cast<unsigned>(y);
+    return months(d <= 11 ? d : d + 12);
+}
+
+CONSTCD14
+inline
+month
+operator+(const month& x, const months& y) NOEXCEPT
+{
+    auto const mu = static_cast<long long>(static_cast<unsigned>(x)) + y.count() - 1;
+    auto const yr = (mu >= 0 ? mu : mu-11) / 12;
+    return month{static_cast<unsigned>(mu - yr * 12 + 1)};
+}
+
+CONSTCD14
+inline
+month
+operator+(const months& x, const month& y) NOEXCEPT
+{
+    return y + x;
+}
+
+CONSTCD14
+inline
+month
+operator-(const month& x, const months& y) NOEXCEPT
+{
+    return x + -y;
+}
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const month& m)
+{
+    if (m.ok())
+    {
+        CharT fmt[] = {'%', 'b', 0};
+        os << format(os.getloc(), fmt, m);
+    }
+    else
+        os << static_cast<unsigned>(m);
+    return os;
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const month& m)
+{
+    detail::low_level_fmt(os, m);
+    if (!m.ok())
+        os << " is not a valid month";
+    return os;
+}
+
+// year
+
+CONSTCD11 inline year::year(int y) NOEXCEPT : y_(static_cast<decltype(y_)>(y)) {}
+CONSTCD14 inline year& year::operator++() NOEXCEPT {++y_; return *this;}
+CONSTCD14 inline year year::operator++(int) NOEXCEPT {auto tmp(*this); ++(*this); return tmp;}
+CONSTCD14 inline year& year::operator--() NOEXCEPT {--y_; return *this;}
+CONSTCD14 inline year year::operator--(int) NOEXCEPT {auto tmp(*this); --(*this); return tmp;}
+CONSTCD14 inline year& year::operator+=(const years& y) NOEXCEPT {*this = *this + y; return *this;}
+CONSTCD14 inline year& year::operator-=(const years& y) NOEXCEPT {*this = *this - y; return *this;}
+CONSTCD11 inline year year::operator-() const NOEXCEPT {return year{-y_};}
+CONSTCD11 inline year year::operator+() const NOEXCEPT {return *this;}
+
+CONSTCD11
+inline
+bool
+year::is_leap() const NOEXCEPT
+{
+    return y_ % 4 == 0 && (y_ % 100 != 0 || y_ % 400 == 0);
+}
+
+CONSTCD11 inline year::operator int() const NOEXCEPT {return y_;}
+
+CONSTCD11
+inline
+bool
+year::ok() const NOEXCEPT
+{
+    return y_ != std::numeric_limits<short>::min();
+}
+
+CONSTCD11
+inline
+bool
+operator==(const year& x, const year& y) NOEXCEPT
+{
+    return static_cast<int>(x) == static_cast<int>(y);
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const year& x, const year& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+CONSTCD11
+inline
+bool
+operator<(const year& x, const year& y) NOEXCEPT
+{
+    return static_cast<int>(x) < static_cast<int>(y);
+}
+
+CONSTCD11
+inline
+bool
+operator>(const year& x, const year& y) NOEXCEPT
+{
+    return y < x;
+}
+
+CONSTCD11
+inline
+bool
+operator<=(const year& x, const year& y) NOEXCEPT
+{
+    return !(y < x);
+}
+
+CONSTCD11
+inline
+bool
+operator>=(const year& x, const year& y) NOEXCEPT
+{
+    return !(x < y);
+}
+
+CONSTCD11
+inline
+years
+operator-(const year& x, const year& y) NOEXCEPT
+{
+    return years{static_cast<int>(x) - static_cast<int>(y)};
+}
+
+CONSTCD11
+inline
+year
+operator+(const year& x, const years& y) NOEXCEPT
+{
+    return year{static_cast<int>(x) + y.count()};
+}
+
+CONSTCD11
+inline
+year
+operator+(const years& x, const year& y) NOEXCEPT
+{
+    return y + x;
+}
+
+CONSTCD11
+inline
+year
+operator-(const year& x, const years& y) NOEXCEPT
+{
+    return year{static_cast<int>(x) - y.count()};
+}
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const year& y)
+{
+    detail::save_ostream<CharT, Traits> _(os);
+    os.fill('0');
+    os.flags(std::ios::dec | std::ios::internal);
+    os.width(4 + (y < year{0}));
+    os.imbue(std::locale::classic());
+    os << static_cast<int>(y);
+    return os;
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year& y)
+{
+    detail::low_level_fmt(os, y);
+    if (!y.ok())
+        os << " is not a valid year";
+    return os;
+}
+
+// weekday
+
+CONSTCD14
+inline
+unsigned char
+weekday::weekday_from_days(int z) NOEXCEPT
+{
+    auto u = static_cast<unsigned>(z);
+    return static_cast<unsigned char>(z >= -4 ? (u+4) % 7 : u % 7);
+}
+
+CONSTCD11
+inline
+weekday::weekday(unsigned wd) NOEXCEPT
+    : wd_(static_cast<decltype(wd_)>(wd != 7 ? wd : 0))
+    {}
+
+CONSTCD14
+inline
+weekday::weekday(const sys_days& dp) NOEXCEPT
+    : wd_(weekday_from_days(dp.time_since_epoch().count()))
+    {}
+
+CONSTCD14
+inline
+weekday::weekday(const local_days& dp) NOEXCEPT
+    : wd_(weekday_from_days(dp.time_since_epoch().count()))
+    {}
+
+CONSTCD14 inline weekday& weekday::operator++() NOEXCEPT {*this += days{1}; return *this;}
+CONSTCD14 inline weekday weekday::operator++(int) NOEXCEPT {auto tmp(*this); ++(*this); return tmp;}
+CONSTCD14 inline weekday& weekday::operator--() NOEXCEPT {*this -= days{1}; return *this;}
+CONSTCD14 inline weekday weekday::operator--(int) NOEXCEPT {auto tmp(*this); --(*this); return tmp;}
+
+CONSTCD14
+inline
+weekday&
+weekday::operator+=(const days& d) NOEXCEPT
+{
+    *this = *this + d;
+    return *this;
+}
+
+CONSTCD14
+inline
+weekday&
+weekday::operator-=(const days& d) NOEXCEPT
+{
+    *this = *this - d;
+    return *this;
+}
+
+CONSTCD11 inline bool weekday::ok() const NOEXCEPT {return wd_ <= 6;}
+
+CONSTCD11
+inline
+unsigned weekday::c_encoding() const NOEXCEPT
+{
+    return unsigned{wd_};
+}
+
+CONSTCD11
+inline
+unsigned weekday::iso_encoding() const NOEXCEPT
+{
+    return unsigned{((wd_ == 0u) ? 7u : wd_)};
+}
+
+CONSTCD11
+inline
+bool
+operator==(const weekday& x, const weekday& y) NOEXCEPT
+{
+    return x.wd_ == y.wd_;
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const weekday& x, const weekday& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+CONSTCD14
+inline
+days
+operator-(const weekday& x, const weekday& y) NOEXCEPT
+{
+    auto const wdu = x.wd_ - y.wd_;
+    auto const wk = (wdu >= 0 ? wdu : wdu-6) / 7;
+    return days{wdu - wk * 7};
+}
+
+CONSTCD14
+inline
+weekday
+operator+(const weekday& x, const days& y) NOEXCEPT
+{
+    auto const wdu = static_cast<long long>(static_cast<unsigned>(x.wd_)) + y.count();
+    auto const wk = (wdu >= 0 ? wdu : wdu-6) / 7;
+    return weekday{static_cast<unsigned>(wdu - wk * 7)};
+}
+
+CONSTCD14
+inline
+weekday
+operator+(const days& x, const weekday& y) NOEXCEPT
+{
+    return y + x;
+}
+
+CONSTCD14
+inline
+weekday
+operator-(const weekday& x, const days& y) NOEXCEPT
+{
+    return x + -y;
+}
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const weekday& wd)
+{
+    if (wd.ok())
+    {
+        CharT fmt[] = {'%', 'a', 0};
+        os << format(fmt, wd);
+    }
+    else
+        os << wd.c_encoding();
+    return os;
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const weekday& wd)
+{
+    detail::low_level_fmt(os, wd);
+    if (!wd.ok())
+        os << " is not a valid weekday";
+    return os;
+}
+
+#if !defined(_MSC_VER) || (_MSC_VER >= 1900)
+inline namespace literals
+{
+
+CONSTCD11
+inline
+date::day
+operator "" _d(unsigned long long d) NOEXCEPT
+{
+    return date::day{static_cast<unsigned>(d)};
+}
+
+CONSTCD11
+inline
+date::year
+operator "" _y(unsigned long long y) NOEXCEPT
+{
+    return date::year(static_cast<int>(y));
+}
+#endif  // !defined(_MSC_VER) || (_MSC_VER >= 1900)
+
+CONSTDATA date::last_spec last{};
+
+CONSTDATA date::month jan{1};
+CONSTDATA date::month feb{2};
+CONSTDATA date::month mar{3};
+CONSTDATA date::month apr{4};
+CONSTDATA date::month may{5};
+CONSTDATA date::month jun{6};
+CONSTDATA date::month jul{7};
+CONSTDATA date::month aug{8};
+CONSTDATA date::month sep{9};
+CONSTDATA date::month oct{10};
+CONSTDATA date::month nov{11};
+CONSTDATA date::month dec{12};
+
+CONSTDATA date::weekday sun{0u};
+CONSTDATA date::weekday mon{1u};
+CONSTDATA date::weekday tue{2u};
+CONSTDATA date::weekday wed{3u};
+CONSTDATA date::weekday thu{4u};
+CONSTDATA date::weekday fri{5u};
+CONSTDATA date::weekday sat{6u};
+
+#if !defined(_MSC_VER) || (_MSC_VER >= 1900)
+}  // inline namespace literals
+#endif
+
+CONSTDATA date::month January{1};
+CONSTDATA date::month February{2};
+CONSTDATA date::month March{3};
+CONSTDATA date::month April{4};
+CONSTDATA date::month May{5};
+CONSTDATA date::month June{6};
+CONSTDATA date::month July{7};
+CONSTDATA date::month August{8};
+CONSTDATA date::month September{9};
+CONSTDATA date::month October{10};
+CONSTDATA date::month November{11};
+CONSTDATA date::month December{12};
+
+CONSTDATA date::weekday Monday{1};
+CONSTDATA date::weekday Tuesday{2};
+CONSTDATA date::weekday Wednesday{3};
+CONSTDATA date::weekday Thursday{4};
+CONSTDATA date::weekday Friday{5};
+CONSTDATA date::weekday Saturday{6};
+CONSTDATA date::weekday Sunday{7};
+
+// weekday_indexed
+
+CONSTCD11
+inline
+weekday
+weekday_indexed::weekday() const NOEXCEPT
+{
+    return date::weekday{static_cast<unsigned>(wd_)};
+}
+
+CONSTCD11 inline unsigned weekday_indexed::index() const NOEXCEPT {return index_;}
+
+CONSTCD11
+inline
+bool
+weekday_indexed::ok() const NOEXCEPT
+{
+    return weekday().ok() && 1 <= index_ && index_ <= 5;
+}
+
+#ifdef __GNUC__
+#  pragma GCC diagnostic push
+#  pragma GCC diagnostic ignored "-Wconversion"
+#endif  // __GNUC__
+
+CONSTCD11
+inline
+weekday_indexed::weekday_indexed(const date::weekday& wd, unsigned index) NOEXCEPT
+    : wd_(static_cast<decltype(wd_)>(static_cast<unsigned>(wd.wd_)))
+    , index_(static_cast<decltype(index_)>(index))
+    {}
+
+#ifdef __GNUC__
+#  pragma GCC diagnostic pop
+#endif  // __GNUC__
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const weekday_indexed& wdi)
+{
+    return low_level_fmt(os, wdi.weekday()) << '[' << wdi.index() << ']';
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const weekday_indexed& wdi)
+{
+    detail::low_level_fmt(os, wdi);
+    if (!wdi.ok())
+        os << " is not a valid weekday_indexed";
+    return os;
+}
+
+CONSTCD11
+inline
+weekday_indexed
+weekday::operator[](unsigned index) const NOEXCEPT
+{
+    return {*this, index};
+}
+
+CONSTCD11
+inline
+bool
+operator==(const weekday_indexed& x, const weekday_indexed& y) NOEXCEPT
+{
+    return x.weekday() == y.weekday() && x.index() == y.index();
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const weekday_indexed& x, const weekday_indexed& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+// weekday_last
+
+CONSTCD11 inline date::weekday weekday_last::weekday() const NOEXCEPT {return wd_;}
+CONSTCD11 inline bool weekday_last::ok() const NOEXCEPT {return wd_.ok();}
+CONSTCD11 inline weekday_last::weekday_last(const date::weekday& wd) NOEXCEPT : wd_(wd) {}
+
+CONSTCD11
+inline
+bool
+operator==(const weekday_last& x, const weekday_last& y) NOEXCEPT
+{
+    return x.weekday() == y.weekday();
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const weekday_last& x, const weekday_last& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const weekday_last& wdl)
+{
+    return low_level_fmt(os, wdl.weekday()) << "[last]";
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const weekday_last& wdl)
+{
+    detail::low_level_fmt(os, wdl);
+    if (!wdl.ok())
+        os << " is not a valid weekday_last";
+    return os;
+}
+
+CONSTCD11
+inline
+weekday_last
+weekday::operator[](last_spec) const NOEXCEPT
+{
+    return weekday_last{*this};
+}
+
+// year_month
+
+CONSTCD11
+inline
+year_month::year_month(const date::year& y, const date::month& m) NOEXCEPT
+    : y_(y)
+    , m_(m)
+    {}
+
+CONSTCD11 inline year year_month::year() const NOEXCEPT {return y_;}
+CONSTCD11 inline month year_month::month() const NOEXCEPT {return m_;}
+CONSTCD11 inline bool year_month::ok() const NOEXCEPT {return y_.ok() && m_.ok();}
+
+template<class>
+CONSTCD14
+inline
+year_month&
+year_month::operator+=(const months& dm) NOEXCEPT
+{
+    *this = *this + dm;
+    return *this;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month&
+year_month::operator-=(const months& dm) NOEXCEPT
+{
+    *this = *this - dm;
+    return *this;
+}
+
+CONSTCD14
+inline
+year_month&
+year_month::operator+=(const years& dy) NOEXCEPT
+{
+    *this = *this + dy;
+    return *this;
+}
+
+CONSTCD14
+inline
+year_month&
+year_month::operator-=(const years& dy) NOEXCEPT
+{
+    *this = *this - dy;
+    return *this;
+}
+
+CONSTCD11
+inline
+bool
+operator==(const year_month& x, const year_month& y) NOEXCEPT
+{
+    return x.year() == y.year() && x.month() == y.month();
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const year_month& x, const year_month& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+CONSTCD11
+inline
+bool
+operator<(const year_month& x, const year_month& y) NOEXCEPT
+{
+    return x.year() < y.year() ? true
+        : (x.year() > y.year() ? false
+        : (x.month() < y.month()));
+}
+
+CONSTCD11
+inline
+bool
+operator>(const year_month& x, const year_month& y) NOEXCEPT
+{
+    return y < x;
+}
+
+CONSTCD11
+inline
+bool
+operator<=(const year_month& x, const year_month& y) NOEXCEPT
+{
+    return !(y < x);
+}
+
+CONSTCD11
+inline
+bool
+operator>=(const year_month& x, const year_month& y) NOEXCEPT
+{
+    return !(x < y);
+}
+
+template<class>
+CONSTCD14
+inline
+year_month
+operator+(const year_month& ym, const months& dm) NOEXCEPT
+{
+    auto dmi = static_cast<int>(static_cast<unsigned>(ym.month())) - 1 + dm.count();
+    auto dy = (dmi >= 0 ? dmi : dmi-11) / 12;
+    dmi = dmi - dy * 12 + 1;
+    return (ym.year() + years(dy)) / month(static_cast<unsigned>(dmi));
+}
+
+template<class>
+CONSTCD14
+inline
+year_month
+operator+(const months& dm, const year_month& ym) NOEXCEPT
+{
+    return ym + dm;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month
+operator-(const year_month& ym, const months& dm) NOEXCEPT
+{
+    return ym + -dm;
+}
+
+CONSTCD11
+inline
+months
+operator-(const year_month& x, const year_month& y) NOEXCEPT
+{
+    return (x.year() - y.year()) +
+            months(static_cast<unsigned>(x.month()) - static_cast<unsigned>(y.month()));
+}
+
+CONSTCD11
+inline
+year_month
+operator+(const year_month& ym, const years& dy) NOEXCEPT
+{
+    return (ym.year() + dy) / ym.month();
+}
+
+CONSTCD11
+inline
+year_month
+operator+(const years& dy, const year_month& ym) NOEXCEPT
+{
+    return ym + dy;
+}
+
+CONSTCD11
+inline
+year_month
+operator-(const year_month& ym, const years& dy) NOEXCEPT
+{
+    return ym + -dy;
+}
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const year_month& ym)
+{
+    low_level_fmt(os, ym.year()) << '/';
+    return low_level_fmt(os, ym.month());
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year_month& ym)
+{
+    detail::low_level_fmt(os, ym);
+    if (!ym.ok())
+        os << " is not a valid year_month";
+    return os;
+}
+
+// month_day
+
+CONSTCD11
+inline
+month_day::month_day(const date::month& m, const date::day& d) NOEXCEPT
+    : m_(m)
+    , d_(d)
+    {}
+
+CONSTCD11 inline date::month month_day::month() const NOEXCEPT {return m_;}
+CONSTCD11 inline date::day month_day::day() const NOEXCEPT {return d_;}
+
+CONSTCD14
+inline
+bool
+month_day::ok() const NOEXCEPT
+{
+    CONSTDATA date::day d[] =
+    {
+        date::day(31), date::day(29), date::day(31),
+        date::day(30), date::day(31), date::day(30),
+        date::day(31), date::day(31), date::day(30),
+        date::day(31), date::day(30), date::day(31)
+    };
+    return m_.ok() && date::day{1} <= d_ && d_ <= d[static_cast<unsigned>(m_)-1];
+}
+
+CONSTCD11
+inline
+bool
+operator==(const month_day& x, const month_day& y) NOEXCEPT
+{
+    return x.month() == y.month() && x.day() == y.day();
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const month_day& x, const month_day& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+CONSTCD11
+inline
+bool
+operator<(const month_day& x, const month_day& y) NOEXCEPT
+{
+    return x.month() < y.month() ? true
+        : (x.month() > y.month() ? false
+        : (x.day() < y.day()));
+}
+
+CONSTCD11
+inline
+bool
+operator>(const month_day& x, const month_day& y) NOEXCEPT
+{
+    return y < x;
+}
+
+CONSTCD11
+inline
+bool
+operator<=(const month_day& x, const month_day& y) NOEXCEPT
+{
+    return !(y < x);
+}
+
+CONSTCD11
+inline
+bool
+operator>=(const month_day& x, const month_day& y) NOEXCEPT
+{
+    return !(x < y);
+}
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const month_day& md)
+{
+    low_level_fmt(os, md.month()) << '/';
+    return low_level_fmt(os, md.day());
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const month_day& md)
+{
+    detail::low_level_fmt(os, md);
+    if (!md.ok())
+        os << " is not a valid month_day";
+    return os;
+}
+
+// month_day_last
+
+CONSTCD11 inline month month_day_last::month() const NOEXCEPT {return m_;}
+CONSTCD11 inline bool month_day_last::ok() const NOEXCEPT {return m_.ok();}
+CONSTCD11 inline month_day_last::month_day_last(const date::month& m) NOEXCEPT : m_(m) {}
+
+CONSTCD11
+inline
+bool
+operator==(const month_day_last& x, const month_day_last& y) NOEXCEPT
+{
+    return x.month() == y.month();
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const month_day_last& x, const month_day_last& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+CONSTCD11
+inline
+bool
+operator<(const month_day_last& x, const month_day_last& y) NOEXCEPT
+{
+    return x.month() < y.month();
+}
+
+CONSTCD11
+inline
+bool
+operator>(const month_day_last& x, const month_day_last& y) NOEXCEPT
+{
+    return y < x;
+}
+
+CONSTCD11
+inline
+bool
+operator<=(const month_day_last& x, const month_day_last& y) NOEXCEPT
+{
+    return !(y < x);
+}
+
+CONSTCD11
+inline
+bool
+operator>=(const month_day_last& x, const month_day_last& y) NOEXCEPT
+{
+    return !(x < y);
+}
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const month_day_last& mdl)
+{
+    return low_level_fmt(os, mdl.month()) << "/last";
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const month_day_last& mdl)
+{
+    detail::low_level_fmt(os, mdl);
+    if (!mdl.ok())
+        os << " is not a valid month_day_last";
+    return os;
+}
+
+// month_weekday
+
+CONSTCD11
+inline
+month_weekday::month_weekday(const date::month& m,
+                             const date::weekday_indexed& wdi) NOEXCEPT
+    : m_(m)
+    , wdi_(wdi)
+    {}
+
+CONSTCD11 inline month month_weekday::month() const NOEXCEPT {return m_;}
+
+CONSTCD11
+inline
+weekday_indexed
+month_weekday::weekday_indexed() const NOEXCEPT
+{
+    return wdi_;
+}
+
+CONSTCD11
+inline
+bool
+month_weekday::ok() const NOEXCEPT
+{
+    return m_.ok() && wdi_.ok();
+}
+
+CONSTCD11
+inline
+bool
+operator==(const month_weekday& x, const month_weekday& y) NOEXCEPT
+{
+    return x.month() == y.month() && x.weekday_indexed() == y.weekday_indexed();
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const month_weekday& x, const month_weekday& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const month_weekday& mwd)
+{
+    low_level_fmt(os, mwd.month()) << '/';
+    return low_level_fmt(os, mwd.weekday_indexed());
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const month_weekday& mwd)
+{
+    detail::low_level_fmt(os, mwd);
+    if (!mwd.ok())
+        os << " is not a valid month_weekday";
+    return os;
+}
+
+// month_weekday_last
+
+CONSTCD11
+inline
+month_weekday_last::month_weekday_last(const date::month& m,
+                                       const date::weekday_last& wdl) NOEXCEPT
+    : m_(m)
+    , wdl_(wdl)
+    {}
+
+CONSTCD11 inline month month_weekday_last::month() const NOEXCEPT {return m_;}
+
+CONSTCD11
+inline
+weekday_last
+month_weekday_last::weekday_last() const NOEXCEPT
+{
+    return wdl_;
+}
+
+CONSTCD11
+inline
+bool
+month_weekday_last::ok() const NOEXCEPT
+{
+    return m_.ok() && wdl_.ok();
+}
+
+CONSTCD11
+inline
+bool
+operator==(const month_weekday_last& x, const month_weekday_last& y) NOEXCEPT
+{
+    return x.month() == y.month() && x.weekday_last() == y.weekday_last();
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const month_weekday_last& x, const month_weekday_last& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const month_weekday_last& mwdl)
+{
+    low_level_fmt(os, mwdl.month()) << '/';
+    return low_level_fmt(os, mwdl.weekday_last());
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const month_weekday_last& mwdl)
+{
+    detail::low_level_fmt(os, mwdl);
+    if (!mwdl.ok())
+        os << " is not a valid month_weekday_last";
+    return os;
+}
+
+// year_month_day_last
+
+CONSTCD11
+inline
+year_month_day_last::year_month_day_last(const date::year& y,
+                                         const date::month_day_last& mdl) NOEXCEPT
+    : y_(y)
+    , mdl_(mdl)
+    {}
+
+template<class>
+CONSTCD14
+inline
+year_month_day_last&
+year_month_day_last::operator+=(const months& m) NOEXCEPT
+{
+    *this = *this + m;
+    return *this;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_day_last&
+year_month_day_last::operator-=(const months& m) NOEXCEPT
+{
+    *this = *this - m;
+    return *this;
+}
+
+CONSTCD14
+inline
+year_month_day_last&
+year_month_day_last::operator+=(const years& y) NOEXCEPT
+{
+    *this = *this + y;
+    return *this;
+}
+
+CONSTCD14
+inline
+year_month_day_last&
+year_month_day_last::operator-=(const years& y) NOEXCEPT
+{
+    *this = *this - y;
+    return *this;
+}
+
+CONSTCD11 inline year year_month_day_last::year() const NOEXCEPT {return y_;}
+CONSTCD11 inline month year_month_day_last::month() const NOEXCEPT {return mdl_.month();}
+
+CONSTCD11
+inline
+month_day_last
+year_month_day_last::month_day_last() const NOEXCEPT
+{
+    return mdl_;
+}
+
+CONSTCD14
+inline
+day
+year_month_day_last::day() const NOEXCEPT
+{
+    CONSTDATA date::day d[] =
+    {
+        date::day(31), date::day(28), date::day(31),
+        date::day(30), date::day(31), date::day(30),
+        date::day(31), date::day(31), date::day(30),
+        date::day(31), date::day(30), date::day(31)
+    };
+    return (month() != February || !y_.is_leap()) && mdl_.ok() ?
+        d[static_cast<unsigned>(month()) - 1] : date::day{29};
+}
+
+CONSTCD14
+inline
+year_month_day_last::operator sys_days() const NOEXCEPT
+{
+    return sys_days(year()/month()/day());
+}
+
+CONSTCD14
+inline
+year_month_day_last::operator local_days() const NOEXCEPT
+{
+    return local_days(year()/month()/day());
+}
+
+CONSTCD11
+inline
+bool
+year_month_day_last::ok() const NOEXCEPT
+{
+    return y_.ok() && mdl_.ok();
+}
+
+CONSTCD11
+inline
+bool
+operator==(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT
+{
+    return x.year() == y.year() && x.month_day_last() == y.month_day_last();
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+CONSTCD11
+inline
+bool
+operator<(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT
+{
+    return x.year() < y.year() ? true
+        : (x.year() > y.year() ? false
+        : (x.month_day_last() < y.month_day_last()));
+}
+
+CONSTCD11
+inline
+bool
+operator>(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT
+{
+    return y < x;
+}
+
+CONSTCD11
+inline
+bool
+operator<=(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT
+{
+    return !(y < x);
+}
+
+CONSTCD11
+inline
+bool
+operator>=(const year_month_day_last& x, const year_month_day_last& y) NOEXCEPT
+{
+    return !(x < y);
+}
+
+namespace detail
+{
+
+template<class CharT, class Traits>
+std::basic_ostream<CharT, Traits>&
+low_level_fmt(std::basic_ostream<CharT, Traits>& os, const year_month_day_last& ymdl)
+{
+    low_level_fmt(os, ymdl.year()) << '/';
+    return low_level_fmt(os, ymdl.month_day_last());
+}
+
+}  // namespace detail
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_day_last& ymdl)
+{
+    detail::low_level_fmt(os, ymdl);
+    if (!ymdl.ok())
+        os << " is not a valid year_month_day_last";
+    return os;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_day_last
+operator+(const year_month_day_last& ymdl, const months& dm) NOEXCEPT
+{
+    return (ymdl.year() / ymdl.month() + dm) / last;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_day_last
+operator+(const months& dm, const year_month_day_last& ymdl) NOEXCEPT
+{
+    return ymdl + dm;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_day_last
+operator-(const year_month_day_last& ymdl, const months& dm) NOEXCEPT
+{
+    return ymdl + (-dm);
+}
+
+CONSTCD11
+inline
+year_month_day_last
+operator+(const year_month_day_last& ymdl, const years& dy) NOEXCEPT
+{
+    return {ymdl.year()+dy, ymdl.month_day_last()};
+}
+
+CONSTCD11
+inline
+year_month_day_last
+operator+(const years& dy, const year_month_day_last& ymdl) NOEXCEPT
+{
+    return ymdl + dy;
+}
+
+CONSTCD11
+inline
+year_month_day_last
+operator-(const year_month_day_last& ymdl, const years& dy) NOEXCEPT
+{
+    return ymdl + (-dy);
+}
+
+// year_month_day
+
+CONSTCD11
+inline
+year_month_day::year_month_day(const date::year& y, const date::month& m,
+                               const date::day& d) NOEXCEPT
+    : y_(y)
+    , m_(m)
+    , d_(d)
+    {}
+
+CONSTCD14
+inline
+year_month_day::year_month_day(const year_month_day_last& ymdl) NOEXCEPT
+    : y_(ymdl.year())
+    , m_(ymdl.month())
+    , d_(ymdl.day())
+    {}
+
+CONSTCD14
+inline
+year_month_day::year_month_day(sys_days dp) NOEXCEPT
+    : year_month_day(from_days(dp.time_since_epoch()))
+    {}
+
+CONSTCD14
+inline
+year_month_day::year_month_day(local_days dp) NOEXCEPT
+    : year_month_day(from_days(dp.time_since_epoch()))
+    {}
+
+CONSTCD11 inline year year_month_day::year() const NOEXCEPT {return y_;}
+CONSTCD11 inline month year_month_day::month() const NOEXCEPT {return m_;}
+CONSTCD11 inline day year_month_day::day() const NOEXCEPT {return d_;}
+
+template<class>
+CONSTCD14
+inline
+year_month_day&
+year_month_day::operator+=(const months& m) NOEXCEPT
+{
+    *this = *this + m;
+    return *this;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_day&
+year_month_day::operator-=(const months& m) NOEXCEPT
+{
+    *this = *this - m;
+    return *this;
+}
+
+CONSTCD14
+inline
+year_month_day&
+year_month_day::operator+=(const years& y) NOEXCEPT
+{
+    *this = *this + y;
+    return *this;
+}
+
+CONSTCD14
+inline
+year_month_day&
+year_month_day::operator-=(const years& y) NOEXCEPT
+{
+    *this = *this - y;
+    return *this;
+}
+
+CONSTCD14
+inline
+days
+year_month_day::to_days() const NOEXCEPT
+{
+    static_assert(std::numeric_limits<unsigned>::digits >= 18,
+             "This algorithm has not been ported to a 16 bit unsigned integer");
+    static_assert(std::numeric_limits<int>::digits >= 20,
+             "This algorithm has not been ported to a 16 bit signed integer");
+    auto const y = static_cast<int>(y_) - (m_ <= February);
+    auto const m = static_cast<unsigned>(m_);
+    auto const d = static_cast<unsigned>(d_);
+    auto const era = (y >= 0 ? y : y-399) / 400;
+    auto const yoe = static_cast<unsigned>(y - era * 400);       // [0, 399]
+    auto const doy = (153*(m > 2 ? m-3 : m+9) + 2)/5 + d-1;      // [0, 365]
+    auto const doe = yoe * 365 + yoe/4 - yoe/100 + doy;          // [0, 146096]
+    return days{era * 146097 + static_cast<int>(doe) - 719468};
+}
+
+CONSTCD14
+inline
+year_month_day::operator sys_days() const NOEXCEPT
+{
+    return sys_days{to_days()};
+}
+
+CONSTCD14
+inline
+year_month_day::operator local_days() const NOEXCEPT
+{
+    return local_days{to_days()};
+}
+
+CONSTCD14
+inline
+bool
+year_month_day::ok() const NOEXCEPT
+{
+    if (!(y_.ok() && m_.ok()))
+        return false;
+    return date::day{1} <= d_ && d_ <= (y_ / m_ / last).day();
+}
+
+CONSTCD11
+inline
+bool
+operator==(const year_month_day& x, const year_month_day& y) NOEXCEPT
+{
+    return x.year() == y.year() && x.month() == y.month() && x.day() == y.day();
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const year_month_day& x, const year_month_day& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+CONSTCD11
+inline
+bool
+operator<(const year_month_day& x, const year_month_day& y) NOEXCEPT
+{
+    return x.year() < y.year() ? true
+        : (x.year() > y.year() ? false
+        : (x.month() < y.month() ? true
+        : (x.month() > y.month() ? false
+        : (x.day() < y.day()))));
+}
+
+CONSTCD11
+inline
+bool
+operator>(const year_month_day& x, const year_month_day& y) NOEXCEPT
+{
+    return y < x;
+}
+
+CONSTCD11
+inline
+bool
+operator<=(const year_month_day& x, const year_month_day& y) NOEXCEPT
+{
+    return !(y < x);
+}
+
+CONSTCD11
+inline
+bool
+operator>=(const year_month_day& x, const year_month_day& y) NOEXCEPT
+{
+    return !(x < y);
+}
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_day& ymd)
+{
+    detail::save_ostream<CharT, Traits> _(os);
+    os.fill('0');
+    os.flags(std::ios::dec | std::ios::right);
+    os.imbue(std::locale::classic());
+    os << static_cast<int>(ymd.year()) << '-';
+    os.width(2);
+    os << static_cast<unsigned>(ymd.month()) << '-';
+    os.width(2);
+    os << static_cast<unsigned>(ymd.day());
+    if (!ymd.ok())
+        os << " is not a valid year_month_day";
+    return os;
+}
+
+CONSTCD14
+inline
+year_month_day
+year_month_day::from_days(days dp) NOEXCEPT
+{
+    static_assert(std::numeric_limits<unsigned>::digits >= 18,
+             "This algorithm has not been ported to a 16 bit unsigned integer");
+    static_assert(std::numeric_limits<int>::digits >= 20,
+             "This algorithm has not been ported to a 16 bit signed integer");
+    auto const z = dp.count() + 719468;
+    auto const era = (z >= 0 ? z : z - 146096) / 146097;
+    auto const doe = static_cast<unsigned>(z - era * 146097);          // [0, 146096]
+    auto const yoe = (doe - doe/1460 + doe/36524 - doe/146096) / 365;  // [0, 399]
+    auto const y = static_cast<days::rep>(yoe) + era * 400;
+    auto const doy = doe - (365*yoe + yoe/4 - yoe/100);                // [0, 365]
+    auto const mp = (5*doy + 2)/153;                                   // [0, 11]
+    auto const d = doy - (153*mp+2)/5 + 1;                             // [1, 31]
+    auto const m = mp < 10 ? mp+3 : mp-9;                              // [1, 12]
+    return year_month_day{date::year{y + (m <= 2)}, date::month(m), date::day(d)};
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_day
+operator+(const year_month_day& ymd, const months& dm) NOEXCEPT
+{
+    return (ymd.year() / ymd.month() + dm) / ymd.day();
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_day
+operator+(const months& dm, const year_month_day& ymd) NOEXCEPT
+{
+    return ymd + dm;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_day
+operator-(const year_month_day& ymd, const months& dm) NOEXCEPT
+{
+    return ymd + (-dm);
+}
+
+CONSTCD11
+inline
+year_month_day
+operator+(const year_month_day& ymd, const years& dy) NOEXCEPT
+{
+    return (ymd.year() + dy) / ymd.month() / ymd.day();
+}
+
+CONSTCD11
+inline
+year_month_day
+operator+(const years& dy, const year_month_day& ymd) NOEXCEPT
+{
+    return ymd + dy;
+}
+
+CONSTCD11
+inline
+year_month_day
+operator-(const year_month_day& ymd, const years& dy) NOEXCEPT
+{
+    return ymd + (-dy);
+}
+
+// year_month_weekday
+
+CONSTCD11
+inline
+year_month_weekday::year_month_weekday(const date::year& y, const date::month& m,
+                                       const date::weekday_indexed& wdi)
+        NOEXCEPT
+    : y_(y)
+    , m_(m)
+    , wdi_(wdi)
+    {}
+
+CONSTCD14
+inline
+year_month_weekday::year_month_weekday(const sys_days& dp) NOEXCEPT
+    : year_month_weekday(from_days(dp.time_since_epoch()))
+    {}
+
+CONSTCD14
+inline
+year_month_weekday::year_month_weekday(const local_days& dp) NOEXCEPT
+    : year_month_weekday(from_days(dp.time_since_epoch()))
+    {}
+
+template<class>
+CONSTCD14
+inline
+year_month_weekday&
+year_month_weekday::operator+=(const months& m) NOEXCEPT
+{
+    *this = *this + m;
+    return *this;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_weekday&
+year_month_weekday::operator-=(const months& m) NOEXCEPT
+{
+    *this = *this - m;
+    return *this;
+}
+
+CONSTCD14
+inline
+year_month_weekday&
+year_month_weekday::operator+=(const years& y) NOEXCEPT
+{
+    *this = *this + y;
+    return *this;
+}
+
+CONSTCD14
+inline
+year_month_weekday&
+year_month_weekday::operator-=(const years& y) NOEXCEPT
+{
+    *this = *this - y;
+    return *this;
+}
+
+CONSTCD11 inline year year_month_weekday::year() const NOEXCEPT {return y_;}
+CONSTCD11 inline month year_month_weekday::month() const NOEXCEPT {return m_;}
+
+CONSTCD11
+inline
+weekday
+year_month_weekday::weekday() const NOEXCEPT
+{
+    return wdi_.weekday();
+}
+
+CONSTCD11
+inline
+unsigned
+year_month_weekday::index() const NOEXCEPT
+{
+    return wdi_.index();
+}
+
+CONSTCD11
+inline
+weekday_indexed
+year_month_weekday::weekday_indexed() const NOEXCEPT
+{
+    return wdi_;
+}
+
+CONSTCD14
+inline
+year_month_weekday::operator sys_days() const NOEXCEPT
+{
+    return sys_days{to_days()};
+}
+
+CONSTCD14
+inline
+year_month_weekday::operator local_days() const NOEXCEPT
+{
+    return local_days{to_days()};
+}
+
+CONSTCD14
+inline
+bool
+year_month_weekday::ok() const NOEXCEPT
+{
+    if (!y_.ok() || !m_.ok() || !wdi_.weekday().ok() || wdi_.index() < 1)
+        return false;
+    if (wdi_.index() <= 4)
+        return true;
+    auto d2 = wdi_.weekday() - date::weekday(static_cast<sys_days>(y_/m_/1)) +
+                  days((wdi_.index()-1)*7 + 1);
+    return static_cast<unsigned>(d2.count()) <= static_cast<unsigned>((y_/m_/last).day());
+}
+
+CONSTCD14
+inline
+year_month_weekday
+year_month_weekday::from_days(days d) NOEXCEPT
+{
+    sys_days dp{d};
+    auto const wd = date::weekday(dp);
+    auto const ymd = year_month_day(dp);
+    return {ymd.year(), ymd.month(), wd[(static_cast<unsigned>(ymd.day())-1)/7+1]};
+}
+
+CONSTCD14
+inline
+days
+year_month_weekday::to_days() const NOEXCEPT
+{
+    auto d = sys_days(y_/m_/1);
+    return (d + (wdi_.weekday() - date::weekday(d) + days{(wdi_.index()-1)*7})
+           ).time_since_epoch();
+}
+
+CONSTCD11
+inline
+bool
+operator==(const year_month_weekday& x, const year_month_weekday& y) NOEXCEPT
+{
+    return x.year() == y.year() && x.month() == y.month() &&
+           x.weekday_indexed() == y.weekday_indexed();
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const year_month_weekday& x, const year_month_weekday& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_weekday& ymwdi)
+{
+    detail::low_level_fmt(os, ymwdi.year()) << '/';
+    detail::low_level_fmt(os, ymwdi.month()) << '/';
+    detail::low_level_fmt(os, ymwdi.weekday_indexed());
+    if (!ymwdi.ok())
+        os << " is not a valid year_month_weekday";
+    return os;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_weekday
+operator+(const year_month_weekday& ymwd, const months& dm) NOEXCEPT
+{
+    return (ymwd.year() / ymwd.month() + dm) / ymwd.weekday_indexed();
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_weekday
+operator+(const months& dm, const year_month_weekday& ymwd) NOEXCEPT
+{
+    return ymwd + dm;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_weekday
+operator-(const year_month_weekday& ymwd, const months& dm) NOEXCEPT
+{
+    return ymwd + (-dm);
+}
+
+CONSTCD11
+inline
+year_month_weekday
+operator+(const year_month_weekday& ymwd, const years& dy) NOEXCEPT
+{
+    return {ymwd.year()+dy, ymwd.month(), ymwd.weekday_indexed()};
+}
+
+CONSTCD11
+inline
+year_month_weekday
+operator+(const years& dy, const year_month_weekday& ymwd) NOEXCEPT
+{
+    return ymwd + dy;
+}
+
+CONSTCD11
+inline
+year_month_weekday
+operator-(const year_month_weekday& ymwd, const years& dy) NOEXCEPT
+{
+    return ymwd + (-dy);
+}
+
+// year_month_weekday_last
+
+CONSTCD11
+inline
+year_month_weekday_last::year_month_weekday_last(const date::year& y,
+                                                 const date::month& m,
+                                                 const date::weekday_last& wdl) NOEXCEPT
+    : y_(y)
+    , m_(m)
+    , wdl_(wdl)
+    {}
+
+template<class>
+CONSTCD14
+inline
+year_month_weekday_last&
+year_month_weekday_last::operator+=(const months& m) NOEXCEPT
+{
+    *this = *this + m;
+    return *this;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_weekday_last&
+year_month_weekday_last::operator-=(const months& m) NOEXCEPT
+{
+    *this = *this - m;
+    return *this;
+}
+
+CONSTCD14
+inline
+year_month_weekday_last&
+year_month_weekday_last::operator+=(const years& y) NOEXCEPT
+{
+    *this = *this + y;
+    return *this;
+}
+
+CONSTCD14
+inline
+year_month_weekday_last&
+year_month_weekday_last::operator-=(const years& y) NOEXCEPT
+{
+    *this = *this - y;
+    return *this;
+}
+
+CONSTCD11 inline year year_month_weekday_last::year() const NOEXCEPT {return y_;}
+CONSTCD11 inline month year_month_weekday_last::month() const NOEXCEPT {return m_;}
+
+CONSTCD11
+inline
+weekday
+year_month_weekday_last::weekday() const NOEXCEPT
+{
+    return wdl_.weekday();
+}
+
+CONSTCD11
+inline
+weekday_last
+year_month_weekday_last::weekday_last() const NOEXCEPT
+{
+    return wdl_;
+}
+
+CONSTCD14
+inline
+year_month_weekday_last::operator sys_days() const NOEXCEPT
+{
+    return sys_days{to_days()};
+}
+
+CONSTCD14
+inline
+year_month_weekday_last::operator local_days() const NOEXCEPT
+{
+    return local_days{to_days()};
+}
+
+CONSTCD11
+inline
+bool
+year_month_weekday_last::ok() const NOEXCEPT
+{
+    return y_.ok() && m_.ok() && wdl_.ok();
+}
+
+CONSTCD14
+inline
+days
+year_month_weekday_last::to_days() const NOEXCEPT
+{
+    auto const d = sys_days(y_/m_/last);
+    return (d - (date::weekday{d} - wdl_.weekday())).time_since_epoch();
+}
+
+CONSTCD11
+inline
+bool
+operator==(const year_month_weekday_last& x, const year_month_weekday_last& y) NOEXCEPT
+{
+    return x.year() == y.year() && x.month() == y.month() &&
+           x.weekday_last() == y.weekday_last();
+}
+
+CONSTCD11
+inline
+bool
+operator!=(const year_month_weekday_last& x, const year_month_weekday_last& y) NOEXCEPT
+{
+    return !(x == y);
+}
+
+template<class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const year_month_weekday_last& ymwdl)
+{
+    detail::low_level_fmt(os, ymwdl.year()) << '/';
+    detail::low_level_fmt(os, ymwdl.month()) << '/';
+    detail::low_level_fmt(os, ymwdl.weekday_last());
+    if (!ymwdl.ok())
+        os << " is not a valid year_month_weekday_last";
+    return os;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_weekday_last
+operator+(const year_month_weekday_last& ymwdl, const months& dm) NOEXCEPT
+{
+    return (ymwdl.year() / ymwdl.month() + dm) / ymwdl.weekday_last();
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_weekday_last
+operator+(const months& dm, const year_month_weekday_last& ymwdl) NOEXCEPT
+{
+    return ymwdl + dm;
+}
+
+template<class>
+CONSTCD14
+inline
+year_month_weekday_last
+operator-(const year_month_weekday_last& ymwdl, const months& dm) NOEXCEPT
+{
+    return ymwdl + (-dm);
+}
+
+CONSTCD11
+inline
+year_month_weekday_last
+operator+(const year_month_weekday_last& ymwdl, const years& dy) NOEXCEPT
+{
+    return {ymwdl.year()+dy, ymwdl.month(), ymwdl.weekday_last()};
+}
+
+CONSTCD11
+inline
+year_month_weekday_last
+operator+(const years& dy, const year_month_weekday_last& ymwdl) NOEXCEPT
+{
+    return ymwdl + dy;
+}
+
+CONSTCD11
+inline
+year_month_weekday_last
+operator-(const year_month_weekday_last& ymwdl, const years& dy) NOEXCEPT
+{
+    return ymwdl + (-dy);
+}
+
+// year_month from operator/()
+
+CONSTCD11
+inline
+year_month
+operator/(const year& y, const month& m) NOEXCEPT
+{
+    return {y, m};
+}
+
+CONSTCD11
+inline
+year_month
+operator/(const year& y, int   m) NOEXCEPT
+{
+    return y / month(static_cast<unsigned>(m));
+}
+
+// month_day from operator/()
+
+CONSTCD11
+inline
+month_day
+operator/(const month& m, const day& d) NOEXCEPT
+{
+    return {m, d};
+}
+
+CONSTCD11
+inline
+month_day
+operator/(const day& d, const month& m) NOEXCEPT
+{
+    return m / d;
+}
+
+CONSTCD11
+inline
+month_day
+operator/(const month& m, int d) NOEXCEPT
+{
+    return m / day(static_cast<unsigned>(d));
+}
+
+CONSTCD11
+inline
+month_day
+operator/(int m, const day& d) NOEXCEPT
+{
+    return month(static_cast<unsigned>(m)) / d;
+}
+
+CONSTCD11 inline month_day operator/(const day& d, int m) NOEXCEPT {return m / d;}
+
+// month_day_last from operator/()
+
+CONSTCD11
+inline
+month_day_last
+operator/(const month& m, last_spec) NOEXCEPT
+{
+    return month_day_last{m};
+}
+
+CONSTCD11
+inline
+month_day_last
+operator/(last_spec, const month& m) NOEXCEPT
+{
+    return m/last;
+}
+
+CONSTCD11
+inline
+month_day_last
+operator/(int m, last_spec) NOEXCEPT
+{
+    return month(static_cast<unsigned>(m))/last;
+}
+
+CONSTCD11
+inline
+month_day_last
+operator/(last_spec, int m) NOEXCEPT
+{
+    return m/last;
+}
+
+// month_weekday from operator/()
+
+CONSTCD11
+inline
+month_weekday
+operator/(const month& m, const weekday_indexed& wdi) NOEXCEPT
+{
+    return {m, wdi};
+}
+
+CONSTCD11
+inline
+month_weekday
+operator/(const weekday_indexed& wdi, const month& m) NOEXCEPT
+{
+    return m / wdi;
+}
+
+CONSTCD11
+inline
+month_weekday
+operator/(int m, const weekday_indexed& wdi) NOEXCEPT
+{
+    return month(static_cast<unsigned>(m)) / wdi;
+}
+
+CONSTCD11
+inline
+month_weekday
+operator/(const weekday_indexed& wdi, int m) NOEXCEPT
+{
+    return m / wdi;
+}
+
+// month_weekday_last from operator/()
+
+CONSTCD11
+inline
+month_weekday_last
+operator/(const month& m, const weekday_last& wdl) NOEXCEPT
+{
+    return {m, wdl};
+}
+
+CONSTCD11
+inline
+month_weekday_last
+operator/(const weekday_last& wdl, const month& m) NOEXCEPT
+{
+    return m / wdl;
+}
+
+CONSTCD11
+inline
+month_weekday_last
+operator/(int m, const weekday_last& wdl) NOEXCEPT
+{
+    return month(static_cast<unsigned>(m)) / wdl;
+}
+
+CONSTCD11
+inline
+month_weekday_last
+operator/(const weekday_last& wdl, int m) NOEXCEPT
+{
+    return m / wdl;
+}
+
+// year_month_day from operator/()
+
+CONSTCD11
+inline
+year_month_day
+operator/(const year_month& ym, const day& d) NOEXCEPT
+{
+    return {ym.year(), ym.month(), d};
+}
+
+CONSTCD11
+inline
+year_month_day
+operator/(const year_month& ym, int d)  NOEXCEPT
+{
+    return ym / day(static_cast<unsigned>(d));
+}
+
+CONSTCD11
+inline
+year_month_day
+operator/(const year& y, const month_day& md) NOEXCEPT
+{
+    return y / md.month() / md.day();
+}
+
+CONSTCD11
+inline
+year_month_day
+operator/(int y, const month_day& md) NOEXCEPT
+{
+    return year(y) / md;
+}
+
+CONSTCD11
+inline
+year_month_day
+operator/(const month_day& md, const year& y)  NOEXCEPT
+{
+    return y / md;
+}
+
+CONSTCD11
+inline
+year_month_day
+operator/(const month_day& md, int y) NOEXCEPT
+{
+    return year(y) / md;
+}
+
+// year_month_day_last from operator/()
+
+CONSTCD11
+inline
+year_month_day_last
+operator/(const year_month& ym, last_spec) NOEXCEPT
+{
+    return {ym.year(), month_day_last{ym.month()}};
+}
+
+CONSTCD11
+inline
+year_month_day_last
+operator/(const year& y, const month_day_last& mdl) NOEXCEPT
+{
+    return {y, mdl};
+}
+
+CONSTCD11
+inline
+year_month_day_last
+operator/(int y, const month_day_last& mdl) NOEXCEPT
+{
+    return year(y) / mdl;
+}
+
+CONSTCD11
+inline
+year_month_day_last
+operator/(const month_day_last& mdl, const year& y) NOEXCEPT
+{
+    return y / mdl;
+}
+
+CONSTCD11
+inline
+year_month_day_last
+operator/(const month_day_last& mdl, int y) NOEXCEPT
+{
+    return year(y) / mdl;
+}
+
+// year_month_weekday from operator/()
+
+CONSTCD11
+inline
+year_month_weekday
+operator/(const year_month& ym, const weekday_indexed& wdi) NOEXCEPT
+{
+    return {ym.year(), ym.month(), wdi};
+}
+
+CONSTCD11
+inline
+year_month_weekday
+operator/(const year& y, const month_weekday& mwd) NOEXCEPT
+{
+    return {y, mwd.month(), mwd.weekday_indexed()};
+}
+
+CONSTCD11
+inline
+year_month_weekday
+operator/(int y, const month_weekday& mwd) NOEXCEPT
+{
+    return year(y) / mwd;
+}
+
+CONSTCD11
+inline
+year_month_weekday
+operator/(const month_weekday& mwd, const year& y) NOEXCEPT
+{
+    return y / mwd;
+}
+
+CONSTCD11
+inline
+year_month_weekday
+operator/(const month_weekday& mwd, int y) NOEXCEPT
+{
+    return year(y) / mwd;
+}
+
+// year_month_weekday_last from operator/()
+
+CONSTCD11
+inline
+year_month_weekday_last
+operator/(const year_month& ym, const weekday_last& wdl) NOEXCEPT
+{
+    return {ym.year(), ym.month(), wdl};
+}
+
+CONSTCD11
+inline
+year_month_weekday_last
+operator/(const year& y, const month_weekday_last& mwdl) NOEXCEPT
+{
+    return {y, mwdl.month(), mwdl.weekday_last()};
+}
+
+CONSTCD11
+inline
+year_month_weekday_last
+operator/(int y, const month_weekday_last& mwdl) NOEXCEPT
+{
+    return year(y) / mwdl;
+}
+
+CONSTCD11
+inline
+year_month_weekday_last
+operator/(const month_weekday_last& mwdl, const year& y) NOEXCEPT
+{
+    return y / mwdl;
+}
+
+CONSTCD11
+inline
+year_month_weekday_last
+operator/(const month_weekday_last& mwdl, int y) NOEXCEPT
+{
+    return year(y) / mwdl;
+}
+
+template <class Duration>
+struct fields;
+
+template <class CharT, class Traits, class Duration>
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt,
+          const fields<Duration>& fds, const std::string* abbrev = nullptr,
+          const std::chrono::seconds* offset_sec = nullptr);
+
+template <class CharT, class Traits, class Duration, class Alloc>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt,
+            fields<Duration>& fds, std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+            std::chrono::minutes* offset = nullptr);
+
+// hh_mm_ss
+
+namespace detail
+{
+
+struct undocumented {explicit undocumented() = default;};
+
+// width<n>::value is the number of fractional decimal digits in 1/n
+// width<0>::value and width<1>::value are defined to be 0
+// If 1/n takes more than 18 fractional decimal digits,
+//   the result is truncated to 19.
+// Example:  width<2>::value    ==  1
+// Example:  width<3>::value    == 19
+// Example:  width<4>::value    ==  2
+// Example:  width<10>::value   ==  1
+// Example:  width<1000>::value ==  3
+template <std::uint64_t n, std::uint64_t d, unsigned w = 0,
+          bool should_continue = n%d != 0 && (w < 19)>
+struct width
+{
+    static_assert(d > 0, "width called with zero denominator");
+    static CONSTDATA unsigned value = 1 + width<n%d*10, d, w+1>::value;
+};
+
+template <std::uint64_t n, std::uint64_t d, unsigned w>
+struct width<n, d, w, false>
+{
+    static CONSTDATA unsigned value = 0;
+};
+
+template <unsigned exp>
+struct static_pow10
+{
+private:
+    static CONSTDATA std::uint64_t h = static_pow10<exp/2>::value;
+public:
+    static CONSTDATA std::uint64_t value = h * h * (exp % 2 ? 10 : 1);
+};
+
+template <>
+struct static_pow10<0>
+{
+    static CONSTDATA std::uint64_t value = 1;
+};
+
+template <class Duration>
+class decimal_format_seconds
+{
+    using CT = typename std::common_type<Duration, std::chrono::seconds>::type;
+    using rep = typename CT::rep;
+    static unsigned CONSTDATA trial_width =
+        detail::width<CT::period::num, CT::period::den>::value;
+public:
+    static unsigned CONSTDATA width = trial_width < 19 ? trial_width : 6u;
+    using precision = std::chrono::duration<rep,
+                                            std::ratio<1, static_pow10<width>::value>>;
+
+private:
+    std::chrono::seconds s_;
+    precision            sub_s_;
+
+public:
+    CONSTCD11 decimal_format_seconds()
+        : s_()
+        , sub_s_()
+        {}
+
+    CONSTCD11 explicit decimal_format_seconds(const Duration& d) NOEXCEPT
+        : s_(std::chrono::duration_cast<std::chrono::seconds>(d))
+        , sub_s_(std::chrono::duration_cast<precision>(d - s_))
+        {}
+
+    CONSTCD14 std::chrono::seconds& seconds() NOEXCEPT {return s_;}
+    CONSTCD11 std::chrono::seconds seconds() const NOEXCEPT {return s_;}
+    CONSTCD11 precision subseconds() const NOEXCEPT {return sub_s_;}
+
+    CONSTCD14 precision to_duration() const NOEXCEPT
+    {
+        return s_ + sub_s_;
+    }
+
+    CONSTCD11 bool in_conventional_range() const NOEXCEPT
+    {
+        return sub_s_ < std::chrono::seconds{1} && s_ < std::chrono::minutes{1};
+    }
+
+    template <class CharT, class Traits>
+    friend
+    std::basic_ostream<CharT, Traits>&
+    operator<<(std::basic_ostream<CharT, Traits>& os, const decimal_format_seconds& x)
+    {
+        return x.print(os, std::chrono::treat_as_floating_point<rep>{});
+    }
+
+    template <class CharT, class Traits>
+    std::basic_ostream<CharT, Traits>&
+    print(std::basic_ostream<CharT, Traits>& os, std::true_type) const
+    {
+        date::detail::save_ostream<CharT, Traits> _(os);
+        std::chrono::duration<rep> d = s_ + sub_s_;
+        if (d < std::chrono::seconds{10})
+            os << '0';
+        os.precision(width+6);
+        os << std::fixed << d.count();
+        return os;
+    }
+
+    template <class CharT, class Traits>
+    std::basic_ostream<CharT, Traits>&
+    print(std::basic_ostream<CharT, Traits>& os, std::false_type) const
+    {
+        date::detail::save_ostream<CharT, Traits> _(os);
+        os.fill('0');
+        os.flags(std::ios::dec | std::ios::right);
+        os.width(2);
+        os << s_.count();
+        if (width > 0)
+        {
+#if !ONLY_C_LOCALE
+            os << std::use_facet<std::numpunct<CharT>>(os.getloc()).decimal_point();
+#else
+            os << '.';
+#endif
+            date::detail::save_ostream<CharT, Traits> _s(os);
+            os.imbue(std::locale::classic());
+            os.width(width);
+            os << sub_s_.count();
+        }
+        return os;
+    }
+};
+
+template <class Rep, class Period>
+inline
+CONSTCD11
+typename std::enable_if
+         <
+            std::numeric_limits<Rep>::is_signed,
+            std::chrono::duration<Rep, Period>
+         >::type
+abs(std::chrono::duration<Rep, Period> d)
+{
+    return d >= d.zero() ? +d : -d;
+}
+
+template <class Rep, class Period>
+inline
+CONSTCD11
+typename std::enable_if
+         <
+            !std::numeric_limits<Rep>::is_signed,
+            std::chrono::duration<Rep, Period>
+         >::type
+abs(std::chrono::duration<Rep, Period> d)
+{
+    return d;
+}
+
+}  // namespace detail
+
+template <class Duration>
+class hh_mm_ss
+{
+    using dfs = detail::decimal_format_seconds<typename std::common_type<Duration,
+                                               std::chrono::seconds>::type>;
+
+    std::chrono::hours h_;
+    std::chrono::minutes m_;
+    dfs s_;
+    bool neg_;
+
+public:
+    static unsigned CONSTDATA fractional_width = dfs::width;
+    using precision = typename dfs::precision;
+
+    CONSTCD11 hh_mm_ss() NOEXCEPT
+        : hh_mm_ss(Duration::zero())
+        {}
+
+    CONSTCD11 explicit hh_mm_ss(Duration d) NOEXCEPT
+        : h_(std::chrono::duration_cast<std::chrono::hours>(detail::abs(d)))
+        , m_(std::chrono::duration_cast<std::chrono::minutes>(detail::abs(d)) - h_)
+        , s_(detail::abs(d) - h_ - m_)
+        , neg_(d < Duration::zero())
+        {}
+
+    CONSTCD11 std::chrono::hours hours() const NOEXCEPT {return h_;}
+    CONSTCD11 std::chrono::minutes minutes() const NOEXCEPT {return m_;}
+    CONSTCD11 std::chrono::seconds seconds() const NOEXCEPT {return s_.seconds();}
+    CONSTCD14 std::chrono::seconds&
+        seconds(detail::undocumented) NOEXCEPT {return s_.seconds();}
+    CONSTCD11 precision subseconds() const NOEXCEPT {return s_.subseconds();}
+    CONSTCD11 bool is_negative() const NOEXCEPT {return neg_;}
+
+    CONSTCD11 explicit operator  precision()   const NOEXCEPT {return to_duration();}
+    CONSTCD11          precision to_duration() const NOEXCEPT
+        {return (s_.to_duration() + m_ + h_) * (1-2*neg_);}
+
+    CONSTCD11 bool in_conventional_range() const NOEXCEPT
+    {
+        return !neg_ && h_ < days{1} && m_ < std::chrono::hours{1} &&
+               s_.in_conventional_range();
+    }
+
+private:
+
+    template <class charT, class traits>
+    friend
+    std::basic_ostream<charT, traits>&
+    operator<<(std::basic_ostream<charT, traits>& os, hh_mm_ss const& tod)
+    {
+        if (tod.is_negative())
+            os << '-';
+        if (tod.h_ < std::chrono::hours{10})
+            os << '0';
+        os << tod.h_.count() << ':';
+        if (tod.m_ < std::chrono::minutes{10})
+            os << '0';
+        os << tod.m_.count() << ':' << tod.s_;
+        return os;
+    }
+
+    template <class CharT, class Traits, class Duration2>
+    friend
+    std::basic_ostream<CharT, Traits>&
+    date::to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt,
+          const fields<Duration2>& fds, const std::string* abbrev,
+          const std::chrono::seconds* offset_sec);
+
+    template <class CharT, class Traits, class Duration2, class Alloc>
+    friend
+    std::basic_istream<CharT, Traits>&
+    date::from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt,
+          fields<Duration2>& fds,
+          std::basic_string<CharT, Traits, Alloc>* abbrev, std::chrono::minutes* offset);
+};
+
+inline
+CONSTCD14
+bool
+is_am(std::chrono::hours const& h) NOEXCEPT
+{
+    using std::chrono::hours;
+    return hours{0} <= h && h < hours{12};
+}
+
+inline
+CONSTCD14
+bool
+is_pm(std::chrono::hours const& h) NOEXCEPT
+{
+    using std::chrono::hours;
+    return hours{12} <= h && h < hours{24};
+}
+
+inline
+CONSTCD14
+std::chrono::hours
+make12(std::chrono::hours h) NOEXCEPT
+{
+    using std::chrono::hours;
+    if (h < hours{12})
+    {
+        if (h == hours{0})
+            h = hours{12};
+    }
+    else
+    {
+        if (h != hours{12})
+            h = h - hours{12};
+    }
+    return h;
+}
+
+inline
+CONSTCD14
+std::chrono::hours
+make24(std::chrono::hours h, bool is_pm) NOEXCEPT
+{
+    using std::chrono::hours;
+    if (is_pm)
+    {
+        if (h != hours{12})
+            h = h + hours{12};
+    }
+    else if (h == hours{12})
+        h = hours{0};
+    return h;
+}
+
+template <class Duration>
+using time_of_day = hh_mm_ss<Duration>;
+
+template <class Rep, class Period>
+CONSTCD11
+inline
+hh_mm_ss<std::chrono::duration<Rep, Period>>
+make_time(const std::chrono::duration<Rep, Period>& d)
+{
+    return hh_mm_ss<std::chrono::duration<Rep, Period>>(d);
+}
+
+template <class CharT, class Traits, class Duration>
+inline
+typename std::enable_if
+<
+    !std::is_convertible<Duration, days>::value,
+    std::basic_ostream<CharT, Traits>&
+>::type
+operator<<(std::basic_ostream<CharT, Traits>& os, const sys_time<Duration>& tp)
+{
+    auto const dp = date::floor<days>(tp);
+    return os << year_month_day(dp) << ' ' << make_time(tp-dp);
+}
+
+template <class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const sys_days& dp)
+{
+    return os << year_month_day(dp);
+}
+
+template <class CharT, class Traits, class Duration>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os, const local_time<Duration>& ut)
+{
+    return (os << sys_time<Duration>{ut.time_since_epoch()});
+}
+
+namespace detail
+{
+
+template <class CharT, std::size_t N>
+class string_literal;
+
+template <class CharT1, class CharT2, std::size_t N1, std::size_t N2>
+inline
+CONSTCD14
+string_literal<typename std::conditional<sizeof(CharT2) <= sizeof(CharT1), CharT1, CharT2>::type,
+               N1 + N2 - 1>
+operator+(const string_literal<CharT1, N1>& x, const string_literal<CharT2, N2>& y) NOEXCEPT;
+
+template <class CharT, std::size_t N>
+class string_literal
+{
+    CharT p_[N];
+
+    CONSTCD11 string_literal() NOEXCEPT
+      : p_{}
+    {}
+
+public:
+    using const_iterator = const CharT*;
+
+    string_literal(string_literal const&) = default;
+    string_literal& operator=(string_literal const&) = delete;
+
+    template <std::size_t N1 = 2,
+              class = typename std::enable_if<N1 == N>::type>
+    CONSTCD11 string_literal(CharT c) NOEXCEPT
+        : p_{c}
+    {
+    }
+
+    template <std::size_t N1 = 3,
+              class = typename std::enable_if<N1 == N>::type>
+    CONSTCD11 string_literal(CharT c1, CharT c2) NOEXCEPT
+        : p_{c1, c2}
+    {
+    }
+
+    template <std::size_t N1 = 4,
+              class = typename std::enable_if<N1 == N>::type>
+    CONSTCD11 string_literal(CharT c1, CharT c2, CharT c3) NOEXCEPT
+        : p_{c1, c2, c3}
+    {
+    }
+
+    CONSTCD14 string_literal(const CharT(&a)[N]) NOEXCEPT
+        : p_{}
+    {
+        for (std::size_t i = 0; i < N; ++i)
+            p_[i] = a[i];
+    }
+
+    template <class U = CharT,
+              class = typename std::enable_if<(1 < sizeof(U))>::type>
+    CONSTCD14 string_literal(const char(&a)[N]) NOEXCEPT
+        : p_{}
+    {
+        for (std::size_t i = 0; i < N; ++i)
+            p_[i] = a[i];
+    }
+
+    template <class CharT2,
+              class = typename std::enable_if<!std::is_same<CharT2, CharT>::value>::type>
+    CONSTCD14 string_literal(string_literal<CharT2, N> const& a) NOEXCEPT
+        : p_{}
+    {
+        for (std::size_t i = 0; i < N; ++i)
+            p_[i] = a[i];
+    }
+
+    CONSTCD11 const CharT* data() const NOEXCEPT {return p_;}
+    CONSTCD11 std::size_t size() const NOEXCEPT {return N-1;}
+
+    CONSTCD11 const_iterator begin() const NOEXCEPT {return p_;}
+    CONSTCD11 const_iterator end()   const NOEXCEPT {return p_ + N-1;}
+
+    CONSTCD11 CharT const& operator[](std::size_t n) const NOEXCEPT
+    {
+        return p_[n];
+    }
+
+    template <class Traits>
+    friend
+    std::basic_ostream<CharT, Traits>&
+    operator<<(std::basic_ostream<CharT, Traits>& os, const string_literal& s)
+    {
+        return os << s.p_;
+    }
+
+    template <class CharT1, class CharT2, std::size_t N1, std::size_t N2>
+    friend
+    CONSTCD14
+    string_literal<typename std::conditional<sizeof(CharT2) <= sizeof(CharT1), CharT1, CharT2>::type,
+                   N1 + N2 - 1>
+    operator+(const string_literal<CharT1, N1>& x, const string_literal<CharT2, N2>& y) NOEXCEPT;
+};
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 3>
+operator+(const string_literal<CharT, 2>& x, const string_literal<CharT, 2>& y) NOEXCEPT
+{
+  return string_literal<CharT, 3>(x[0], y[0]);
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 4>
+operator+(const string_literal<CharT, 3>& x, const string_literal<CharT, 2>& y) NOEXCEPT
+{
+  return string_literal<CharT, 4>(x[0], x[1], y[0]);
+}
+
+template <class CharT1, class CharT2, std::size_t N1, std::size_t N2>
+CONSTCD14
+inline
+string_literal<typename std::conditional<sizeof(CharT2) <= sizeof(CharT1), CharT1, CharT2>::type,
+               N1 + N2 - 1>
+operator+(const string_literal<CharT1, N1>& x, const string_literal<CharT2, N2>& y) NOEXCEPT
+{
+    using CT = typename std::conditional<sizeof(CharT2) <= sizeof(CharT1), CharT1, CharT2>::type;
+
+    string_literal<CT, N1 + N2 - 1> r;
+    std::size_t i = 0;
+    for (; i < N1-1; ++i)
+       r.p_[i] = CT(x.p_[i]);
+    for (std::size_t j = 0; j < N2; ++j, ++i)
+       r.p_[i] = CT(y.p_[j]);
+
+    return r;
+}
+
+
+template <class CharT, class Traits, class Alloc, std::size_t N>
+inline
+std::basic_string<CharT, Traits, Alloc>
+operator+(std::basic_string<CharT, Traits, Alloc> x, const string_literal<CharT, N>& y)
+{
+    x.append(y.data(), y.size());
+    return x;
+}
+
+#if __cplusplus >= 201402  && (!defined(__EDG_VERSION__) || __EDG_VERSION__ > 411) \
+                           && (!defined(__SUNPRO_CC) || __SUNPRO_CC > 0x5150)
+
+template <class CharT,
+          class = std::enable_if_t<std::is_same<CharT, char>::value ||
+                                   std::is_same<CharT, wchar_t>::value ||
+                                   std::is_same<CharT, char16_t>::value ||
+                                   std::is_same<CharT, char32_t>::value>>
+CONSTCD14
+inline
+string_literal<CharT, 2>
+msl(CharT c) NOEXCEPT
+{
+    return string_literal<CharT, 2>{c};
+}
+
+CONSTCD14
+inline
+std::size_t
+to_string_len(std::intmax_t i)
+{
+    std::size_t r = 0;
+    do
+    {
+        i /= 10;
+        ++r;
+    } while (i > 0);
+    return r;
+}
+
+template <std::intmax_t N>
+CONSTCD14
+inline
+std::enable_if_t
+<
+    N < 10,
+    string_literal<char, to_string_len(N)+1>
+>
+msl() NOEXCEPT
+{
+    return msl(char(N % 10 + '0'));
+}
+
+template <std::intmax_t N>
+CONSTCD14
+inline
+std::enable_if_t
+<
+    10 <= N,
+    string_literal<char, to_string_len(N)+1>
+>
+msl() NOEXCEPT
+{
+    return msl<N/10>() + msl(char(N % 10 + '0'));
+}
+
+template <class CharT, std::intmax_t N, std::intmax_t D>
+CONSTCD14
+inline
+std::enable_if_t
+<
+    std::ratio<N, D>::type::den != 1,
+    string_literal<CharT, to_string_len(std::ratio<N, D>::type::num) +
+                          to_string_len(std::ratio<N, D>::type::den) + 4>
+>
+msl(std::ratio<N, D>) NOEXCEPT
+{
+    using R = typename std::ratio<N, D>::type;
+    return msl(CharT{'['}) + msl<R::num>() + msl(CharT{'/'}) +
+                             msl<R::den>() + msl(CharT{']'});
+}
+
+template <class CharT, std::intmax_t N, std::intmax_t D>
+CONSTCD14
+inline
+std::enable_if_t
+<
+    std::ratio<N, D>::type::den == 1,
+    string_literal<CharT, to_string_len(std::ratio<N, D>::type::num) + 3>
+>
+msl(std::ratio<N, D>) NOEXCEPT
+{
+    using R = typename std::ratio<N, D>::type;
+    return msl(CharT{'['}) + msl<R::num>() + msl(CharT{']'});
+}
+
+
+#else  // __cplusplus < 201402 || (defined(__EDG_VERSION__) && __EDG_VERSION__ <= 411)
+
+inline
+std::string
+to_string(std::uint64_t x)
+{
+    return std::to_string(x);
+}
+
+template <class CharT>
+inline
+std::basic_string<CharT>
+to_string(std::uint64_t x)
+{
+    auto y = std::to_string(x);
+    return std::basic_string<CharT>(y.begin(), y.end());
+}
+
+template <class CharT, std::intmax_t N, std::intmax_t D>
+inline
+typename std::enable_if
+<
+    std::ratio<N, D>::type::den != 1,
+    std::basic_string<CharT>
+>::type
+msl(std::ratio<N, D>)
+{
+    using R = typename std::ratio<N, D>::type;
+    return std::basic_string<CharT>(1, '[') + to_string<CharT>(R::num) + CharT{'/'} +
+                                              to_string<CharT>(R::den) + CharT{']'};
+}
+
+template <class CharT, std::intmax_t N, std::intmax_t D>
+inline
+typename std::enable_if
+<
+    std::ratio<N, D>::type::den == 1,
+    std::basic_string<CharT>
+>::type
+msl(std::ratio<N, D>)
+{
+    using R = typename std::ratio<N, D>::type;
+    return std::basic_string<CharT>(1, '[') + to_string<CharT>(R::num) + CharT{']'};
+}
+
+#endif  // __cplusplus < 201402 || (defined(__EDG_VERSION__) && __EDG_VERSION__ <= 411)
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::atto) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'a'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::femto) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'f'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::pico) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'p'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::nano) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'n'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+typename std::enable_if
+<
+    std::is_same<CharT, char>::value,
+    string_literal<char, 3>
+>::type
+msl(std::micro) NOEXCEPT
+{
+    return string_literal<char, 3>{'\xC2', '\xB5'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+typename std::enable_if
+<
+    !std::is_same<CharT, char>::value,
+    string_literal<CharT, 2>
+>::type
+msl(std::micro) NOEXCEPT
+{
+    return string_literal<CharT, 2>{CharT{static_cast<unsigned char>('\xB5')}};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::milli) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'m'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::centi) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'c'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 3>
+msl(std::deca) NOEXCEPT
+{
+    return string_literal<CharT, 3>{'d', 'a'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::deci) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'d'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::hecto) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'h'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::kilo) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'k'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::mega) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'M'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::giga) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'G'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::tera) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'T'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::peta) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'P'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+msl(std::exa) NOEXCEPT
+{
+    return string_literal<CharT, 2>{'E'};
+}
+
+template <class CharT, class Period>
+CONSTCD11
+inline
+auto
+get_units(Period p)
+ -> decltype(msl<CharT>(p) + string_literal<CharT, 2>{'s'})
+{
+    return msl<CharT>(p) + string_literal<CharT, 2>{'s'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+get_units(std::ratio<1>)
+{
+    return string_literal<CharT, 2>{'s'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+get_units(std::ratio<3600>)
+{
+    return string_literal<CharT, 2>{'h'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 4>
+get_units(std::ratio<60>)
+{
+    return string_literal<CharT, 4>{'m', 'i', 'n'};
+}
+
+template <class CharT>
+CONSTCD11
+inline
+string_literal<CharT, 2>
+get_units(std::ratio<86400>)
+{
+    return string_literal<CharT, 2>{'d'};
+}
+
+template <class CharT, class Traits = std::char_traits<CharT>>
+struct make_string;
+
+template <>
+struct make_string<char>
+{
+    template <class Rep>
+    static
+    std::string
+    from(Rep n)
+    {
+        return std::to_string(n);
+    }
+};
+
+template <class Traits>
+struct make_string<char, Traits>
+{
+    template <class Rep>
+    static
+    std::basic_string<char, Traits>
+    from(Rep n)
+    {
+        auto s = std::to_string(n);
+        return std::basic_string<char, Traits>(s.begin(), s.end());
+    }
+};
+
+template <>
+struct make_string<wchar_t>
+{
+    template <class Rep>
+    static
+    std::wstring
+    from(Rep n)
+    {
+        return std::to_wstring(n);
+    }
+};
+
+template <class Traits>
+struct make_string<wchar_t, Traits>
+{
+    template <class Rep>
+    static
+    std::basic_string<wchar_t, Traits>
+    from(Rep n)
+    {
+        auto s = std::to_wstring(n);
+        return std::basic_string<wchar_t, Traits>(s.begin(), s.end());
+    }
+};
+
+}  // namespace detail
+
+// to_stream
+
+CONSTDATA year nanyear{-32768};
+
+template <class Duration>
+struct fields
+{
+    year_month_day        ymd{nanyear/0/0};
+    weekday               wd{8u};
+    hh_mm_ss<Duration>    tod{};
+    bool                  has_tod = false;
+
+#if !defined(__clang__) && defined(__GNUC__) && (__GNUC__ * 100 + __GNUC_MINOR__ <= 409)
+    fields() : ymd{nanyear/0/0}, wd{8u}, tod{}, has_tod{false} {}
+#else
+    fields() = default;
+#endif
+
+    fields(year_month_day ymd_) : ymd(ymd_) {}
+    fields(weekday wd_) : wd(wd_) {}
+    fields(hh_mm_ss<Duration> tod_) : tod(tod_), has_tod(true) {}
+
+    fields(year_month_day ymd_, weekday wd_) : ymd(ymd_), wd(wd_) {}
+    fields(year_month_day ymd_, hh_mm_ss<Duration> tod_) : ymd(ymd_), tod(tod_),
+                                                           has_tod(true) {}
+
+    fields(weekday wd_, hh_mm_ss<Duration> tod_) : wd(wd_), tod(tod_), has_tod(true) {}
+
+    fields(year_month_day ymd_, weekday wd_, hh_mm_ss<Duration> tod_)
+        : ymd(ymd_)
+        , wd(wd_)
+        , tod(tod_)
+        , has_tod(true)
+        {}
+};
+
+namespace detail
+{
+
+template <class CharT, class Traits, class Duration>
+unsigned
+extract_weekday(std::basic_ostream<CharT, Traits>& os, const fields<Duration>& fds)
+{
+    if (!fds.ymd.ok() && !fds.wd.ok())
+    {
+        // fds does not contain a valid weekday
+        os.setstate(std::ios::failbit);
+        return 8;
+    }
+    weekday wd;
+    if (fds.ymd.ok())
+    {
+        wd = weekday{sys_days(fds.ymd)};
+        if (fds.wd.ok() && wd != fds.wd)
+        {
+            // fds.ymd and fds.wd are inconsistent
+            os.setstate(std::ios::failbit);
+            return 8;
+        }
+    }
+    else
+        wd = fds.wd;
+    return static_cast<unsigned>((wd - Sunday).count());
+}
+
+template <class CharT, class Traits, class Duration>
+unsigned
+extract_month(std::basic_ostream<CharT, Traits>& os, const fields<Duration>& fds)
+{
+    if (!fds.ymd.month().ok())
+    {
+        // fds does not contain a valid month
+        os.setstate(std::ios::failbit);
+        return 0;
+    }
+    return static_cast<unsigned>(fds.ymd.month());
+}
+
+}  // namespace detail
+
+#if ONLY_C_LOCALE
+
+namespace detail
+{
+
+inline
+std::pair<const std::string*, const std::string*>
+weekday_names()
+{
+    static const std::string nm[] =
+    {
+        "Sunday",
+        "Monday",
+        "Tuesday",
+        "Wednesday",
+        "Thursday",
+        "Friday",
+        "Saturday",
+        "Sun",
+        "Mon",
+        "Tue",
+        "Wed",
+        "Thu",
+        "Fri",
+        "Sat"
+    };
+    return std::make_pair(nm, nm+sizeof(nm)/sizeof(nm[0]));
+}
+
+inline
+std::pair<const std::string*, const std::string*>
+month_names()
+{
+    static const std::string nm[] =
+    {
+        "January",
+        "February",
+        "March",
+        "April",
+        "May",
+        "June",
+        "July",
+        "August",
+        "September",
+        "October",
+        "November",
+        "December",
+        "Jan",
+        "Feb",
+        "Mar",
+        "Apr",
+        "May",
+        "Jun",
+        "Jul",
+        "Aug",
+        "Sep",
+        "Oct",
+        "Nov",
+        "Dec"
+    };
+    return std::make_pair(nm, nm+sizeof(nm)/sizeof(nm[0]));
+}
+
+inline
+std::pair<const std::string*, const std::string*>
+ampm_names()
+{
+    static const std::string nm[] =
+    {
+        "AM",
+        "PM"
+    };
+    return std::make_pair(nm, nm+sizeof(nm)/sizeof(nm[0]));
+}
+
+template <class CharT, class Traits, class FwdIter>
+FwdIter
+scan_keyword(std::basic_istream<CharT, Traits>& is, FwdIter kb, FwdIter ke)
+{
+    size_t nkw = static_cast<size_t>(std::distance(kb, ke));
+    const unsigned char doesnt_match = '\0';
+    const unsigned char might_match = '\1';
+    const unsigned char does_match = '\2';
+    unsigned char statbuf[100];
+    unsigned char* status = statbuf;
+    std::unique_ptr<unsigned char, void(*)(void*)> stat_hold(0, free);
+    if (nkw > sizeof(statbuf))
+    {
+        status = (unsigned char*)std::malloc(nkw);
+        if (status == nullptr)
+            std::terminate();
+        stat_hold.reset(status);
+    }
+    size_t n_might_match = nkw;  // At this point, any keyword might match
+    size_t n_does_match = 0;     // but none of them definitely do
+    // Initialize all statuses to might_match, except for "" keywords are does_match
+    unsigned char* st = status;
+    for (auto ky = kb; ky != ke; ++ky, ++st)
+    {
+        if (!ky->empty())
+            *st = might_match;
+        else
+        {
+            *st = does_match;
+            --n_might_match;
+            ++n_does_match;
+        }
+    }
+    // While there might be a match, test keywords against the next CharT
+    for (size_t indx = 0; is && n_might_match > 0; ++indx)
+    {
+        // Peek at the next CharT but don't consume it
+        auto ic = is.peek();
+        if (ic == EOF)
+        {
+            is.setstate(std::ios::eofbit);
+            break;
+        }
+        auto c = static_cast<char>(toupper(static_cast<unsigned char>(ic)));
+        bool consume = false;
+        // For each keyword which might match, see if the indx character is c
+        // If a match if found, consume c
+        // If a match is found, and that is the last character in the keyword,
+        //    then that keyword matches.
+        // If the keyword doesn't match this character, then change the keyword
+        //    to doesn't match
+        st = status;
+        for (auto ky = kb; ky != ke; ++ky, ++st)
+        {
+            if (*st == might_match)
+            {
+                if (c == static_cast<char>(toupper(static_cast<unsigned char>((*ky)[indx]))))
+                {
+                    consume = true;
+                    if (ky->size() == indx+1)
+                    {
+                        *st = does_match;
+                        --n_might_match;
+                        ++n_does_match;
+                    }
+                }
+                else
+                {
+                    *st = doesnt_match;
+                    --n_might_match;
+                }
+            }
+        }
+        // consume if we matched a character
+        if (consume)
+        {
+            (void)is.get();
+            // If we consumed a character and there might be a matched keyword that
+            //   was marked matched on a previous iteration, then such keywords
+            //   are now marked as not matching.
+            if (n_might_match + n_does_match > 1)
+            {
+                st = status;
+                for (auto ky = kb; ky != ke; ++ky, ++st)
+                {
+                    if (*st == does_match && ky->size() != indx+1)
+                    {
+                        *st = doesnt_match;
+                        --n_does_match;
+                    }
+                }
+            }
+        }
+    }
+    // We've exited the loop because we hit eof and/or we have no more "might matches".
+    // Return the first matching result
+    for (st = status; kb != ke; ++kb, ++st)
+        if (*st == does_match)
+            break;
+    if (kb == ke)
+        is.setstate(std::ios::failbit);
+    return kb;
+}
+
+}  // namespace detail
+
+#endif  // ONLY_C_LOCALE
+
+template <class CharT, class Traits, class Duration>
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt,
+          const fields<Duration>& fds, const std::string* abbrev,
+          const std::chrono::seconds* offset_sec)
+{
+#if ONLY_C_LOCALE
+    using detail::weekday_names;
+    using detail::month_names;
+    using detail::ampm_names;
+#endif
+    using detail::save_ostream;
+    using detail::get_units;
+    using detail::extract_weekday;
+    using detail::extract_month;
+    using std::ios;
+    using std::chrono::duration_cast;
+    using std::chrono::seconds;
+    using std::chrono::minutes;
+    using std::chrono::hours;
+    date::detail::save_ostream<CharT, Traits> ss(os);
+    os.fill(' ');
+    os.flags(std::ios::skipws | std::ios::dec);
+    os.width(0);
+    tm tm{};
+    bool insert_negative = fds.has_tod && fds.tod.to_duration() < Duration::zero();
+#if !ONLY_C_LOCALE
+    auto& facet = std::use_facet<std::time_put<CharT>>(os.getloc());
+#endif
+    const CharT* command = nullptr;
+    CharT modified = CharT{};
+    for (; *fmt; ++fmt)
+    {
+        switch (*fmt)
+        {
+        case 'a':
+        case 'A':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    tm.tm_wday = static_cast<int>(extract_weekday(os, fds));
+                    if (os.fail())
+                        return os;
+#if !ONLY_C_LOCALE
+                    const CharT f[] = {'%', *fmt};
+                    facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+#else  // ONLY_C_LOCALE
+                    os << weekday_names().first[tm.tm_wday+7*(*fmt == 'a')];
+#endif  // ONLY_C_LOCALE
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    modified = CharT{};
+                }
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'b':
+        case 'B':
+        case 'h':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    tm.tm_mon = static_cast<int>(extract_month(os, fds)) - 1;
+#if !ONLY_C_LOCALE
+                    const CharT f[] = {'%', *fmt};
+                    facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+#else  // ONLY_C_LOCALE
+                    os << month_names().first[tm.tm_mon+12*(*fmt != 'B')];
+#endif  // ONLY_C_LOCALE
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    modified = CharT{};
+                }
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'c':
+        case 'x':
+            if (command)
+            {
+                if (modified == CharT{'O'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    if (!fds.ymd.ok())
+                        os.setstate(std::ios::failbit);
+                    if (*fmt == 'c' && !fds.has_tod)
+                        os.setstate(std::ios::failbit);
+#if !ONLY_C_LOCALE
+                    tm = std::tm{};
+                    auto const& ymd = fds.ymd;
+                    auto ld = local_days(ymd);
+                    if (*fmt == 'c')
+                    {
+                        tm.tm_sec = static_cast<int>(fds.tod.seconds().count());
+                        tm.tm_min = static_cast<int>(fds.tod.minutes().count());
+                        tm.tm_hour = static_cast<int>(fds.tod.hours().count());
+                    }
+                    tm.tm_mday = static_cast<int>(static_cast<unsigned>(ymd.day()));
+                    tm.tm_mon = static_cast<int>(extract_month(os, fds) - 1);
+                    tm.tm_year = static_cast<int>(ymd.year()) - 1900;
+                    tm.tm_wday = static_cast<int>(extract_weekday(os, fds));
+                    if (os.fail())
+                        return os;
+                    tm.tm_yday = static_cast<int>((ld - local_days(ymd.year()/1/1)).count());
+                    CharT f[3] = {'%'};
+                    auto fe = std::begin(f) + 1;
+                    if (modified == CharT{'E'})
+                        *fe++ = modified;
+                    *fe++ = *fmt;
+                    facet.put(os, os, os.fill(), &tm, std::begin(f), fe);
+#else  // ONLY_C_LOCALE
+                    if (*fmt == 'c')
+                    {
+                        auto wd = static_cast<int>(extract_weekday(os, fds));
+                        os << weekday_names().first[static_cast<unsigned>(wd)+7]
+                           << ' ';
+                        os << month_names().first[extract_month(os, fds)-1+12] << ' ';
+                        auto d = static_cast<int>(static_cast<unsigned>(fds.ymd.day()));
+                        if (d < 10)
+                            os << ' ';
+                        os << d << ' '
+                           << make_time(duration_cast<seconds>(fds.tod.to_duration()))
+                           << ' ' << fds.ymd.year();
+
+                    }
+                    else  // *fmt == 'x'
+                    {
+                        auto const& ymd = fds.ymd;
+                        save_ostream<CharT, Traits> _(os);
+                        os.fill('0');
+                        os.flags(std::ios::dec | std::ios::right);
+                        os.width(2);
+                        os << static_cast<unsigned>(ymd.month()) << CharT{'/'};
+                        os.width(2);
+                        os << static_cast<unsigned>(ymd.day()) << CharT{'/'};
+                        os.width(2);
+                        os << static_cast<int>(ymd.year()) % 100;
+                    }
+#endif  // ONLY_C_LOCALE
+                }
+                command = nullptr;
+                modified = CharT{};
+            }
+            else
+                os << *fmt;
+            break;
+        case 'C':
+            if (command)
+            {
+                if (modified == CharT{'O'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    if (!fds.ymd.year().ok())
+                        os.setstate(std::ios::failbit);
+                    auto y = static_cast<int>(fds.ymd.year());
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        save_ostream<CharT, Traits> _(os);
+                        os.fill('0');
+                        os.flags(std::ios::dec | std::ios::right);
+                        if (y >= 0)
+                        {
+                            os.width(2);
+                            os << y/100;
+                        }
+                        else
+                        {
+                            os << CharT{'-'};
+                            os.width(2);
+                            os << -(y-99)/100;
+                        }
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'E'})
+                    {
+                        tm.tm_year = y - 1900;
+                        CharT f[3] = {'%', 'E', 'C'};
+                        facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                    }
+#endif
+                }
+                command = nullptr;
+                modified = CharT{};
+            }
+            else
+                os << *fmt;
+            break;
+        case 'd':
+        case 'e':
+            if (command)
+            {
+                if (modified == CharT{'E'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    if (!fds.ymd.day().ok())
+                        os.setstate(std::ios::failbit);
+                    auto d = static_cast<int>(static_cast<unsigned>(fds.ymd.day()));
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        save_ostream<CharT, Traits> _(os);
+                        if (*fmt == CharT{'d'})
+                            os.fill('0');
+                        else
+                            os.fill(' ');
+                        os.flags(std::ios::dec | std::ios::right);
+                        os.width(2);
+                        os << d;
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        tm.tm_mday = d;
+                        CharT f[3] = {'%', 'O', *fmt};
+                        facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                    }
+#endif
+                }
+                command = nullptr;
+                modified = CharT{};
+            }
+            else
+                os << *fmt;
+            break;
+        case 'D':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    if (!fds.ymd.ok())
+                        os.setstate(std::ios::failbit);
+                    auto const& ymd = fds.ymd;
+                    save_ostream<CharT, Traits> _(os);
+                    os.fill('0');
+                    os.flags(std::ios::dec | std::ios::right);
+                    os.width(2);
+                    os << static_cast<unsigned>(ymd.month()) << CharT{'/'};
+                    os.width(2);
+                    os << static_cast<unsigned>(ymd.day()) << CharT{'/'};
+                    os.width(2);
+                    os << static_cast<int>(ymd.year()) % 100;
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    modified = CharT{};
+                }
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'F':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    if (!fds.ymd.ok())
+                        os.setstate(std::ios::failbit);
+                    auto const& ymd = fds.ymd;
+                    save_ostream<CharT, Traits> _(os);
+                    os.imbue(std::locale::classic());
+                    os.fill('0');
+                    os.flags(std::ios::dec | std::ios::right);
+                    os.width(4);
+                    os << static_cast<int>(ymd.year()) << CharT{'-'};
+                    os.width(2);
+                    os << static_cast<unsigned>(ymd.month()) << CharT{'-'};
+                    os.width(2);
+                    os << static_cast<unsigned>(ymd.day());
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    modified = CharT{};
+                }
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'g':
+        case 'G':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    if (!fds.ymd.ok())
+                        os.setstate(std::ios::failbit);
+                    auto ld = local_days(fds.ymd);
+                    auto y = year_month_day{ld + days{3}}.year();
+                    auto start = local_days((y-years{1})/December/Thursday[last]) +
+                                 (Monday-Thursday);
+                    if (ld < start)
+                        --y;
+                    if (*fmt == CharT{'G'})
+                        os << y;
+                    else
+                    {
+                        save_ostream<CharT, Traits> _(os);
+                        os.fill('0');
+                        os.flags(std::ios::dec | std::ios::right);
+                        os.width(2);
+                        os << std::abs(static_cast<int>(y)) % 100;
+                    }
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    modified = CharT{};
+                }
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'H':
+        case 'I':
+            if (command)
+            {
+                if (modified == CharT{'E'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    if (!fds.has_tod)
+                        os.setstate(std::ios::failbit);
+                    if (insert_negative)
+                    {
+                        os << '-';
+                        insert_negative = false;
+                    }
+                    auto hms = fds.tod;
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        auto h = *fmt == CharT{'I'} ? date::make12(hms.hours()) : hms.hours();
+                        if (h < hours{10})
+                            os << CharT{'0'};
+                        os << h.count();
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        const CharT f[] = {'%', modified, *fmt};
+                        tm.tm_hour = static_cast<int>(hms.hours().count());
+                        facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                    }
+#endif
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'j':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    if (fds.ymd.ok() || fds.has_tod)
+                    {
+                        days doy;
+                        if (fds.ymd.ok())
+                        {
+                            auto ld = local_days(fds.ymd);
+                            auto y = fds.ymd.year();
+                            doy = ld - local_days(y/January/1) + days{1};
+                        }
+                        else
+                        {
+                            doy = duration_cast<days>(fds.tod.to_duration());
+                        }
+                        save_ostream<CharT, Traits> _(os);
+                        os.fill('0');
+                        os.flags(std::ios::dec | std::ios::right);
+                        os.width(3);
+                        os << doy.count();
+                    }
+                    else
+                    {
+                        os.setstate(std::ios::failbit);
+                    }
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    modified = CharT{};
+                }
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'm':
+            if (command)
+            {
+                if (modified == CharT{'E'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    if (!fds.ymd.month().ok())
+                        os.setstate(std::ios::failbit);
+                    auto m = static_cast<unsigned>(fds.ymd.month());
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        if (m < 10)
+                            os << CharT{'0'};
+                        os << m;
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        const CharT f[] = {'%', modified, *fmt};
+                        tm.tm_mon = static_cast<int>(m-1);
+                        facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                    }
+#endif
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'M':
+            if (command)
+            {
+                if (modified == CharT{'E'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    if (!fds.has_tod)
+                        os.setstate(std::ios::failbit);
+                    if (insert_negative)
+                    {
+                        os << '-';
+                        insert_negative = false;
+                    }
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        if (fds.tod.minutes() < minutes{10})
+                            os << CharT{'0'};
+                        os << fds.tod.minutes().count();
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        const CharT f[] = {'%', modified, *fmt};
+                        tm.tm_min = static_cast<int>(fds.tod.minutes().count());
+                        facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                    }
+#endif
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'n':
+            if (command)
+            {
+                if (modified == CharT{})
+                    os << CharT{'\n'};
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    modified = CharT{};
+                }
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'p':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    if (!fds.has_tod)
+                        os.setstate(std::ios::failbit);
+#if !ONLY_C_LOCALE
+                    const CharT f[] = {'%', *fmt};
+                    tm.tm_hour = static_cast<int>(fds.tod.hours().count());
+                    facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+#else
+                    if (date::is_am(fds.tod.hours()))
+                        os << ampm_names().first[0];
+                    else
+                        os << ampm_names().first[1];
+#endif
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'Q':
+        case 'q':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    if (!fds.has_tod)
+                        os.setstate(std::ios::failbit);
+                    auto d = fds.tod.to_duration();
+                    if (*fmt == 'q')
+                        os << get_units<CharT>(typename decltype(d)::period::type{});
+                    else
+                        os << d.count();
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'r':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    if (!fds.has_tod)
+                        os.setstate(std::ios::failbit);
+#if !ONLY_C_LOCALE
+                    const CharT f[] = {'%', *fmt};
+                    tm.tm_hour = static_cast<int>(fds.tod.hours().count());
+                    tm.tm_min = static_cast<int>(fds.tod.minutes().count());
+                    tm.tm_sec = static_cast<int>(fds.tod.seconds().count());
+                    facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+#else
+                    hh_mm_ss<seconds> tod(duration_cast<seconds>(fds.tod.to_duration()));
+                    save_ostream<CharT, Traits> _(os);
+                    os.fill('0');
+                    os.width(2);
+                    os << date::make12(tod.hours()).count() << CharT{':'};
+                    os.width(2);
+                    os << tod.minutes().count() << CharT{':'};
+                    os.width(2);
+                    os << tod.seconds().count() << CharT{' '};
+                    if (date::is_am(tod.hours()))
+                        os << ampm_names().first[0];
+                    else
+                        os << ampm_names().first[1];
+#endif
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'R':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    if (!fds.has_tod)
+                        os.setstate(std::ios::failbit);
+                    if (fds.tod.hours() < hours{10})
+                        os << CharT{'0'};
+                    os << fds.tod.hours().count() << CharT{':'};
+                    if (fds.tod.minutes() < minutes{10})
+                        os << CharT{'0'};
+                    os << fds.tod.minutes().count();
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    modified = CharT{};
+                }
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'S':
+            if (command)
+            {
+                if (modified == CharT{'E'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    if (!fds.has_tod)
+                        os.setstate(std::ios::failbit);
+                    if (insert_negative)
+                    {
+                        os << '-';
+                        insert_negative = false;
+                    }
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        os << fds.tod.s_;
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        const CharT f[] = {'%', modified, *fmt};
+                        tm.tm_sec = static_cast<int>(fds.tod.s_.seconds().count());
+                        facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                    }
+#endif
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 't':
+            if (command)
+            {
+                if (modified == CharT{})
+                    os << CharT{'\t'};
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    modified = CharT{};
+                }
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'T':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    if (!fds.has_tod)
+                        os.setstate(std::ios::failbit);
+                    os << fds.tod;
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    modified = CharT{};
+                }
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'u':
+            if (command)
+            {
+                if (modified == CharT{'E'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    auto wd = extract_weekday(os, fds);
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        os << (wd != 0 ? wd : 7u);
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        const CharT f[] = {'%', modified, *fmt};
+                        tm.tm_wday = static_cast<int>(wd);
+                        facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                    }
+#endif
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'U':
+            if (command)
+            {
+                if (modified == CharT{'E'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    auto const& ymd = fds.ymd;
+                    if (!ymd.ok())
+                        os.setstate(std::ios::failbit);
+                    auto ld = local_days(ymd);
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        auto st = local_days(Sunday[1]/January/ymd.year());
+                        if (ld < st)
+                            os << CharT{'0'} << CharT{'0'};
+                        else
+                        {
+                            auto wn = duration_cast<weeks>(ld - st).count() + 1;
+                            if (wn < 10)
+                                os << CharT{'0'};
+                            os << wn;
+                        }
+                   }
+ #if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        const CharT f[] = {'%', modified, *fmt};
+                        tm.tm_year = static_cast<int>(ymd.year()) - 1900;
+                        tm.tm_wday = static_cast<int>(extract_weekday(os, fds));
+                        if (os.fail())
+                            return os;
+                        tm.tm_yday = static_cast<int>((ld - local_days(ymd.year()/1/1)).count());
+                        facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                    }
+#endif
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'V':
+            if (command)
+            {
+                if (modified == CharT{'E'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    if (!fds.ymd.ok())
+                        os.setstate(std::ios::failbit);
+                    auto ld = local_days(fds.ymd);
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        auto y = year_month_day{ld + days{3}}.year();
+                        auto st = local_days((y-years{1})/12/Thursday[last]) +
+                                  (Monday-Thursday);
+                        if (ld < st)
+                        {
+                            --y;
+                            st = local_days((y - years{1})/12/Thursday[last]) +
+                                 (Monday-Thursday);
+                        }
+                        auto wn = duration_cast<weeks>(ld - st).count() + 1;
+                        if (wn < 10)
+                            os << CharT{'0'};
+                        os << wn;
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        const CharT f[] = {'%', modified, *fmt};
+                        auto const& ymd = fds.ymd;
+                        tm.tm_year = static_cast<int>(ymd.year()) - 1900;
+                        tm.tm_wday = static_cast<int>(extract_weekday(os, fds));
+                        if (os.fail())
+                            return os;
+                        tm.tm_yday = static_cast<int>((ld - local_days(ymd.year()/1/1)).count());
+                        facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                    }
+#endif
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'w':
+            if (command)
+            {
+                auto wd = extract_weekday(os, fds);
+                if (os.fail())
+                    return os;
+#if !ONLY_C_LOCALE
+                if (modified == CharT{})
+#else
+                if (modified != CharT{'E'})
+#endif
+                {
+                    os << wd;
+                }
+#if !ONLY_C_LOCALE
+                else if (modified == CharT{'O'})
+                {
+                    const CharT f[] = {'%', modified, *fmt};
+                    tm.tm_wday = static_cast<int>(wd);
+                    facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                }
+#endif
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'W':
+            if (command)
+            {
+                if (modified == CharT{'E'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    auto const& ymd = fds.ymd;
+                    if (!ymd.ok())
+                        os.setstate(std::ios::failbit);
+                    auto ld = local_days(ymd);
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        auto st = local_days(Monday[1]/January/ymd.year());
+                        if (ld < st)
+                            os << CharT{'0'} << CharT{'0'};
+                        else
+                        {
+                            auto wn = duration_cast<weeks>(ld - st).count() + 1;
+                            if (wn < 10)
+                                os << CharT{'0'};
+                            os << wn;
+                        }
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        const CharT f[] = {'%', modified, *fmt};
+                        tm.tm_year = static_cast<int>(ymd.year()) - 1900;
+                        tm.tm_wday = static_cast<int>(extract_weekday(os, fds));
+                        if (os.fail())
+                            return os;
+                        tm.tm_yday = static_cast<int>((ld - local_days(ymd.year()/1/1)).count());
+                        facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                    }
+#endif
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'X':
+            if (command)
+            {
+                if (modified == CharT{'O'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    if (!fds.has_tod)
+                        os.setstate(std::ios::failbit);
+#if !ONLY_C_LOCALE
+                    tm = std::tm{};
+                    tm.tm_sec = static_cast<int>(fds.tod.seconds().count());
+                    tm.tm_min = static_cast<int>(fds.tod.minutes().count());
+                    tm.tm_hour = static_cast<int>(fds.tod.hours().count());
+                    CharT f[3] = {'%'};
+                    auto fe = std::begin(f) + 1;
+                    if (modified == CharT{'E'})
+                        *fe++ = modified;
+                    *fe++ = *fmt;
+                    facet.put(os, os, os.fill(), &tm, std::begin(f), fe);
+#else
+                    os << fds.tod;
+#endif
+                }
+                command = nullptr;
+                modified = CharT{};
+            }
+            else
+                os << *fmt;
+            break;
+        case 'y':
+            if (command)
+            {
+                if (!fds.ymd.year().ok())
+                    os.setstate(std::ios::failbit);
+                auto y = static_cast<int>(fds.ymd.year());
+#if !ONLY_C_LOCALE
+                if (modified == CharT{})
+                {
+#endif
+                    y = std::abs(y) % 100;
+                    if (y < 10)
+                        os << CharT{'0'};
+                    os << y;
+#if !ONLY_C_LOCALE
+                }
+                else
+                {
+                    const CharT f[] = {'%', modified, *fmt};
+                    tm.tm_year = y - 1900;
+                    facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                }
+#endif
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'Y':
+            if (command)
+            {
+                if (modified == CharT{'O'})
+                    os << CharT{'%'} << modified << *fmt;
+                else
+                {
+                    if (!fds.ymd.year().ok())
+                        os.setstate(std::ios::failbit);
+                    auto y = fds.ymd.year();
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        save_ostream<CharT, Traits> _(os);
+                        os.imbue(std::locale::classic());
+                        os << y;
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'E'})
+                    {
+                        const CharT f[] = {'%', modified, *fmt};
+                        tm.tm_year = static_cast<int>(y) - 1900;
+                        facet.put(os, os, os.fill(), &tm, std::begin(f), std::end(f));
+                    }
+#endif
+                }
+                modified = CharT{};
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'z':
+            if (command)
+            {
+                if (offset_sec == nullptr)
+                {
+                    // Can not format %z with unknown offset
+                    os.setstate(ios::failbit);
+                    return os;
+                }
+                auto m = duration_cast<minutes>(*offset_sec);
+                auto neg = m < minutes{0};
+                m = date::abs(m);
+                auto h = duration_cast<hours>(m);
+                m -= h;
+                if (neg)
+                    os << CharT{'-'};
+                else
+                    os << CharT{'+'};
+                if (h < hours{10})
+                    os << CharT{'0'};
+                os << h.count();
+                if (modified != CharT{})
+                    os << CharT{':'};
+                if (m < minutes{10})
+                    os << CharT{'0'};
+                os << m.count();
+                command = nullptr;
+                modified = CharT{};
+            }
+            else
+                os << *fmt;
+            break;
+        case 'Z':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    if (abbrev == nullptr)
+                    {
+                        // Can not format %Z with unknown time_zone
+                        os.setstate(ios::failbit);
+                        return os;
+                    }
+                    for (auto c : *abbrev)
+                        os << CharT(c);
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    modified = CharT{};
+                }
+                command = nullptr;
+            }
+            else
+                os << *fmt;
+            break;
+        case 'E':
+        case 'O':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    modified = *fmt;
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << *fmt;
+                    command = nullptr;
+                    modified = CharT{};
+                }
+            }
+            else
+                os << *fmt;
+            break;
+        case '%':
+            if (command)
+            {
+                if (modified == CharT{})
+                {
+                    os << CharT{'%'};
+                    command = nullptr;
+                }
+                else
+                {
+                    os << CharT{'%'} << modified << CharT{'%'};
+                    command = nullptr;
+                    modified = CharT{};
+                }
+            }
+            else
+                command = fmt;
+            break;
+        default:
+            if (command)
+            {
+                os << CharT{'%'};
+                command = nullptr;
+            }
+            if (modified != CharT{})
+            {
+                os << modified;
+                modified = CharT{};
+            }
+            os << *fmt;
+            break;
+        }
+    }
+    if (command)
+        os << CharT{'%'};
+    if (modified != CharT{})
+        os << modified;
+    return os;
+}
+
+template <class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt, const year& y)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{y/0/0};
+    return to_stream(os, fmt, fds);
+}
+
+template <class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt, const month& m)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{m/0/nanyear};
+    return to_stream(os, fmt, fds);
+}
+
+template <class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt, const day& d)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{d/0/nanyear};
+    return to_stream(os, fmt, fds);
+}
+
+template <class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt, const weekday& wd)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{wd};
+    return to_stream(os, fmt, fds);
+}
+
+template <class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt, const year_month& ym)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{ym/0};
+    return to_stream(os, fmt, fds);
+}
+
+template <class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt, const month_day& md)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{md/nanyear};
+    return to_stream(os, fmt, fds);
+}
+
+template <class CharT, class Traits>
+inline
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt,
+          const year_month_day& ymd)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{ymd};
+    return to_stream(os, fmt, fds);
+}
+
+template <class CharT, class Traits, class Rep, class Period>
+inline
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt,
+          const std::chrono::duration<Rep, Period>& d)
+{
+    using Duration = std::chrono::duration<Rep, Period>;
+    using CT = typename std::common_type<Duration, std::chrono::seconds>::type;
+    fields<CT> fds{hh_mm_ss<CT>{d}};
+    return to_stream(os, fmt, fds);
+}
+
+template <class CharT, class Traits, class Duration>
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt,
+          const local_time<Duration>& tp, const std::string* abbrev = nullptr,
+          const std::chrono::seconds* offset_sec = nullptr)
+{
+    using CT = typename std::common_type<Duration, std::chrono::seconds>::type;
+    auto ld = std::chrono::time_point_cast<days>(tp);
+    fields<CT> fds;
+    if (ld <= tp)
+        fds = fields<CT>{year_month_day{ld}, hh_mm_ss<CT>{tp-local_seconds{ld}}};
+    else
+        fds = fields<CT>{year_month_day{ld - days{1}},
+                         hh_mm_ss<CT>{days{1} - (local_seconds{ld} - tp)}};
+    return to_stream(os, fmt, fds, abbrev, offset_sec);
+}
+
+template <class CharT, class Traits, class Duration>
+std::basic_ostream<CharT, Traits>&
+to_stream(std::basic_ostream<CharT, Traits>& os, const CharT* fmt,
+          const sys_time<Duration>& tp)
+{
+    using std::chrono::seconds;
+    using CT = typename std::common_type<Duration, seconds>::type;
+    const std::string abbrev("UTC");
+    CONSTDATA seconds offset{0};
+    auto sd = std::chrono::time_point_cast<days>(tp);
+    fields<CT> fds;
+    if (sd <= tp)
+        fds = fields<CT>{year_month_day{sd}, hh_mm_ss<CT>{tp-sys_seconds{sd}}};
+    else
+        fds = fields<CT>{year_month_day{sd - days{1}},
+                         hh_mm_ss<CT>{days{1} - (sys_seconds{sd} - tp)}};
+    return to_stream(os, fmt, fds, &abbrev, &offset);
+}
+
+// format
+
+template <class CharT, class Streamable>
+auto
+format(const std::locale& loc, const CharT* fmt, const Streamable& tp)
+    -> decltype(to_stream(std::declval<std::basic_ostream<CharT>&>(), fmt, tp),
+                std::basic_string<CharT>{})
+{
+    std::basic_ostringstream<CharT> os;
+    os.exceptions(std::ios::failbit | std::ios::badbit);
+    os.imbue(loc);
+    to_stream(os, fmt, tp);
+    return os.str();
+}
+
+template <class CharT, class Streamable>
+auto
+format(const CharT* fmt, const Streamable& tp)
+    -> decltype(to_stream(std::declval<std::basic_ostream<CharT>&>(), fmt, tp),
+                std::basic_string<CharT>{})
+{
+    std::basic_ostringstream<CharT> os;
+    os.exceptions(std::ios::failbit | std::ios::badbit);
+    to_stream(os, fmt, tp);
+    return os.str();
+}
+
+template <class CharT, class Traits, class Alloc, class Streamable>
+auto
+format(const std::locale& loc, const std::basic_string<CharT, Traits, Alloc>& fmt,
+       const Streamable& tp)
+    -> decltype(to_stream(std::declval<std::basic_ostream<CharT, Traits>&>(), fmt.c_str(), tp),
+                std::basic_string<CharT, Traits, Alloc>{})
+{
+    std::basic_ostringstream<CharT, Traits, Alloc> os;
+    os.exceptions(std::ios::failbit | std::ios::badbit);
+    os.imbue(loc);
+    to_stream(os, fmt.c_str(), tp);
+    return os.str();
+}
+
+template <class CharT, class Traits, class Alloc, class Streamable>
+auto
+format(const std::basic_string<CharT, Traits, Alloc>& fmt, const Streamable& tp)
+    -> decltype(to_stream(std::declval<std::basic_ostream<CharT, Traits>&>(), fmt.c_str(), tp),
+                std::basic_string<CharT, Traits, Alloc>{})
+{
+    std::basic_ostringstream<CharT, Traits, Alloc> os;
+    os.exceptions(std::ios::failbit | std::ios::badbit);
+    to_stream(os, fmt.c_str(), tp);
+    return os.str();
+}
+
+// parse
+
+namespace detail
+{
+
+template <class CharT, class Traits>
+bool
+read_char(std::basic_istream<CharT, Traits>& is, CharT fmt, std::ios::iostate& err)
+{
+    auto ic = is.get();
+    if (Traits::eq_int_type(ic, Traits::eof()) ||
+       !Traits::eq(Traits::to_char_type(ic), fmt))
+    {
+        err |= std::ios::failbit;
+        is.setstate(std::ios::failbit);
+        return false;
+    }
+    return true;
+}
+
+template <class CharT, class Traits>
+unsigned
+read_unsigned(std::basic_istream<CharT, Traits>& is, unsigned m = 1, unsigned M = 10)
+{
+    unsigned x = 0;
+    unsigned count = 0;
+    while (true)
+    {
+        auto ic = is.peek();
+        if (Traits::eq_int_type(ic, Traits::eof()))
+            break;
+        auto c = static_cast<char>(Traits::to_char_type(ic));
+        if (!('0' <= c && c <= '9'))
+            break;
+        (void)is.get();
+        ++count;
+        x = 10*x + static_cast<unsigned>(c - '0');
+        if (count == M)
+            break;
+    }
+    if (count < m)
+        is.setstate(std::ios::failbit);
+    return x;
+}
+
+template <class CharT, class Traits>
+int
+read_signed(std::basic_istream<CharT, Traits>& is, unsigned m = 1, unsigned M = 10)
+{
+    auto ic = is.peek();
+    if (!Traits::eq_int_type(ic, Traits::eof()))
+    {
+        auto c = static_cast<char>(Traits::to_char_type(ic));
+        if (('0' <= c && c <= '9') || c == '-' || c == '+')
+        {
+            if (c == '-' || c == '+')
+                (void)is.get();
+            auto x = static_cast<int>(read_unsigned(is, std::max(m, 1u), M));
+            if (!is.fail())
+            {
+                if (c == '-')
+                    x = -x;
+                return x;
+            }
+        }
+    }
+    if (m > 0)
+        is.setstate(std::ios::failbit);
+    return 0;
+}
+
+template <class CharT, class Traits>
+long double
+read_long_double(std::basic_istream<CharT, Traits>& is, unsigned m = 1, unsigned M = 10)
+{
+    unsigned count = 0;
+    unsigned fcount = 0;
+    unsigned long long i = 0;
+    unsigned long long f = 0;
+    bool parsing_fraction = false;
+#if ONLY_C_LOCALE
+    typename Traits::int_type decimal_point = '.';
+#else
+    auto decimal_point = Traits::to_int_type(
+        std::use_facet<std::numpunct<CharT>>(is.getloc()).decimal_point());
+#endif
+    while (true)
+    {
+        auto ic = is.peek();
+        if (Traits::eq_int_type(ic, Traits::eof()))
+            break;
+        if (Traits::eq_int_type(ic, decimal_point))
+        {
+            decimal_point = Traits::eof();
+            parsing_fraction = true;
+        }
+        else
+        {
+            auto c = static_cast<char>(Traits::to_char_type(ic));
+            if (!('0' <= c && c <= '9'))
+                break;
+            if (!parsing_fraction)
+            {
+                i = 10*i + static_cast<unsigned>(c - '0');
+            }
+            else
+            {
+                f = 10*f + static_cast<unsigned>(c - '0');
+                ++fcount;
+            }
+        }
+        (void)is.get();
+        if (++count == M)
+            break;
+    }
+    if (count < m)
+    {
+        is.setstate(std::ios::failbit);
+        return 0;
+    }
+    return static_cast<long double>(i) + static_cast<long double>(f)/std::pow(10.L, fcount);
+}
+
+struct rs
+{
+    int& i;
+    unsigned m;
+    unsigned M;
+};
+
+struct ru
+{
+    int& i;
+    unsigned m;
+    unsigned M;
+};
+
+struct rld
+{
+    long double& i;
+    unsigned m;
+    unsigned M;
+};
+
+template <class CharT, class Traits>
+void
+read(std::basic_istream<CharT, Traits>&)
+{
+}
+
+template <class CharT, class Traits, class ...Args>
+void
+read(std::basic_istream<CharT, Traits>& is, CharT a0, Args&& ...args);
+
+template <class CharT, class Traits, class ...Args>
+void
+read(std::basic_istream<CharT, Traits>& is, rs a0, Args&& ...args);
+
+template <class CharT, class Traits, class ...Args>
+void
+read(std::basic_istream<CharT, Traits>& is, ru a0, Args&& ...args);
+
+template <class CharT, class Traits, class ...Args>
+void
+read(std::basic_istream<CharT, Traits>& is, int a0, Args&& ...args);
+
+template <class CharT, class Traits, class ...Args>
+void
+read(std::basic_istream<CharT, Traits>& is, rld a0, Args&& ...args);
+
+template <class CharT, class Traits, class ...Args>
+void
+read(std::basic_istream<CharT, Traits>& is, CharT a0, Args&& ...args)
+{
+    // No-op if a0 == CharT{}
+    if (a0 != CharT{})
+    {
+        auto ic = is.peek();
+        if (Traits::eq_int_type(ic, Traits::eof()))
+        {
+            is.setstate(std::ios::failbit | std::ios::eofbit);
+            return;
+        }
+        if (!Traits::eq(Traits::to_char_type(ic), a0))
+        {
+            is.setstate(std::ios::failbit);
+            return;
+        }
+        (void)is.get();
+    }
+    read(is, std::forward<Args>(args)...);
+}
+
+template <class CharT, class Traits, class ...Args>
+void
+read(std::basic_istream<CharT, Traits>& is, rs a0, Args&& ...args)
+{
+    auto x = read_signed(is, a0.m, a0.M);
+    if (is.fail())
+        return;
+    a0.i = x;
+    read(is, std::forward<Args>(args)...);
+}
+
+template <class CharT, class Traits, class ...Args>
+void
+read(std::basic_istream<CharT, Traits>& is, ru a0, Args&& ...args)
+{
+    auto x = read_unsigned(is, a0.m, a0.M);
+    if (is.fail())
+        return;
+    a0.i = static_cast<int>(x);
+    read(is, std::forward<Args>(args)...);
+}
+
+template <class CharT, class Traits, class ...Args>
+void
+read(std::basic_istream<CharT, Traits>& is, int a0, Args&& ...args)
+{
+    if (a0 != -1)
+    {
+        auto u = static_cast<unsigned>(a0);
+        CharT buf[std::numeric_limits<unsigned>::digits10+2u] = {};
+        auto e = buf;
+        do
+        {
+            *e++ = static_cast<CharT>(CharT(u % 10) + CharT{'0'});
+            u /= 10;
+        } while (u > 0);
+        std::reverse(buf, e);
+        for (auto p = buf; p != e && is.rdstate() == std::ios::goodbit; ++p)
+            read(is, *p);
+    }
+    if (is.rdstate() == std::ios::goodbit)
+        read(is, std::forward<Args>(args)...);
+}
+
+template <class CharT, class Traits, class ...Args>
+void
+read(std::basic_istream<CharT, Traits>& is, rld a0, Args&& ...args)
+{
+    auto x = read_long_double(is, a0.m, a0.M);
+    if (is.fail())
+        return;
+    a0.i = x;
+    read(is, std::forward<Args>(args)...);
+}
+
+template <class T, class CharT, class Traits>
+inline
+void
+checked_set(T& value, T from, T not_a_value, std::basic_ios<CharT, Traits>& is)
+{
+    if (!is.fail())
+    {
+        if (value == not_a_value)
+            value = std::move(from);
+        else if (value != from)
+            is.setstate(std::ios::failbit);
+    }
+}
+
+}  // namespace detail;
+
+template <class CharT, class Traits, class Duration, class Alloc = std::allocator<CharT>>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt,
+            fields<Duration>& fds, std::basic_string<CharT, Traits, Alloc>* abbrev,
+            std::chrono::minutes* offset)
+{
+    using std::numeric_limits;
+    using std::ios;
+    using std::chrono::duration;
+    using std::chrono::duration_cast;
+    using std::chrono::seconds;
+    using std::chrono::minutes;
+    using std::chrono::hours;
+    using detail::round_i;
+    typename std::basic_istream<CharT, Traits>::sentry ok{is, true};
+    if (ok)
+    {
+        date::detail::save_istream<CharT, Traits> ss(is);
+        is.fill(' ');
+        is.flags(std::ios::skipws | std::ios::dec);
+        is.width(0);
+#if !ONLY_C_LOCALE
+        auto& f = std::use_facet<std::time_get<CharT>>(is.getloc());
+        std::tm tm{};
+#endif
+        const CharT* command = nullptr;
+        auto modified = CharT{};
+        auto width = -1;
+
+        CONSTDATA int not_a_year = numeric_limits<short>::min();
+        CONSTDATA int not_a_2digit_year = 100;
+        CONSTDATA int not_a_century = not_a_year / 100;
+        CONSTDATA int not_a_month = 0;
+        CONSTDATA int not_a_day = 0;
+        CONSTDATA int not_a_hour = numeric_limits<int>::min();
+        CONSTDATA int not_a_hour_12_value = 0;
+        CONSTDATA int not_a_minute = not_a_hour;
+        CONSTDATA Duration not_a_second = Duration::min();
+        CONSTDATA int not_a_doy = -1;
+        CONSTDATA int not_a_weekday = 8;
+        CONSTDATA int not_a_week_num = 100;
+        CONSTDATA int not_a_ampm = -1;
+        CONSTDATA minutes not_a_offset = minutes::min();
+
+        int Y = not_a_year;             // c, F, Y                   *
+        int y = not_a_2digit_year;      // D, x, y                   *
+        int g = not_a_2digit_year;      // g                         *
+        int G = not_a_year;             // G                         *
+        int C = not_a_century;          // C                         *
+        int m = not_a_month;            // b, B, h, m, c, D, F, x    *
+        int d = not_a_day;              // c, d, D, e, F, x          *
+        int j = not_a_doy;              // j                         *
+        int wd = not_a_weekday;         // a, A, u, w                *
+        int H = not_a_hour;             // c, H, R, T, X             *
+        int I = not_a_hour_12_value;    // I, r                      *
+        int p = not_a_ampm;             // p, r                      *
+        int M = not_a_minute;           // c, M, r, R, T, X          *
+        Duration s = not_a_second;      // c, r, S, T, X             *
+        int U = not_a_week_num;         // U                         *
+        int V = not_a_week_num;         // V                         *
+        int W = not_a_week_num;         // W                         *
+        std::basic_string<CharT, Traits, Alloc> temp_abbrev;  // Z   *
+        minutes temp_offset = not_a_offset;  // z                    *
+
+        using detail::read;
+        using detail::rs;
+        using detail::ru;
+        using detail::rld;
+        using detail::checked_set;
+        for (; *fmt != CharT{} && !is.fail(); ++fmt)
+        {
+            switch (*fmt)
+            {
+            case 'a':
+            case 'A':
+            case 'u':
+            case 'w':  // wd:  a, A, u, w
+                if (command)
+                {
+                    int trial_wd = not_a_weekday;
+                    if (*fmt == 'a' || *fmt == 'A')
+                    {
+                        if (modified == CharT{})
+                        {
+#if !ONLY_C_LOCALE
+                            ios::iostate err = ios::goodbit;
+                            f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                            is.setstate(err);
+                            if (!is.fail())
+                                trial_wd = tm.tm_wday;
+#else
+                            auto nm = detail::weekday_names();
+                            auto i = detail::scan_keyword(is, nm.first, nm.second) - nm.first;
+                            if (!is.fail())
+                                trial_wd = i % 7;
+#endif
+                        }
+                        else
+                            read(is, CharT{'%'}, width, modified, *fmt);
+                    }
+                    else  // *fmt == 'u' || *fmt == 'w'
+                    {
+#if !ONLY_C_LOCALE
+                        if (modified == CharT{})
+#else
+                        if (modified != CharT{'E'})
+#endif
+                        {
+                            read(is, ru{trial_wd, 1, width == -1 ?
+                                                      1u : static_cast<unsigned>(width)});
+                            if (!is.fail())
+                            {
+                                if (*fmt == 'u')
+                                {
+                                    if (!(1 <= trial_wd && trial_wd <= 7))
+                                    {
+                                        trial_wd = not_a_weekday;
+                                        is.setstate(ios::failbit);
+                                    }
+                                    else if (trial_wd == 7)
+                                        trial_wd = 0;
+                                }
+                                else  // *fmt == 'w'
+                                {
+                                    if (!(0 <= trial_wd && trial_wd <= 6))
+                                    {
+                                        trial_wd = not_a_weekday;
+                                        is.setstate(ios::failbit);
+                                    }
+                                }
+                            }
+                        }
+#if !ONLY_C_LOCALE
+                        else if (modified == CharT{'O'})
+                        {
+                            ios::iostate err = ios::goodbit;
+                            f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                            is.setstate(err);
+                            if (!is.fail())
+                                trial_wd = tm.tm_wday;
+                        }
+#endif
+                        else
+                            read(is, CharT{'%'}, width, modified, *fmt);
+                    }
+                    if (trial_wd != not_a_weekday)
+                        checked_set(wd, trial_wd, not_a_weekday, is);
+                }
+                else  // !command
+                    read(is, *fmt);
+                command = nullptr;
+                width = -1;
+                modified = CharT{};
+                break;
+            case 'b':
+            case 'B':
+            case 'h':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int ttm = not_a_month;
+#if !ONLY_C_LOCALE
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                            ttm = tm.tm_mon + 1;
+                        is.setstate(err);
+#else
+                        auto nm = detail::month_names();
+                        auto i = detail::scan_keyword(is, nm.first, nm.second) - nm.first;
+                        if (!is.fail())
+                            ttm = i % 12 + 1;
+#endif
+                        checked_set(m, ttm, not_a_month, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'c':
+                if (command)
+                {
+                    if (modified != CharT{'O'})
+                    {
+#if !ONLY_C_LOCALE
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                        {
+                            checked_set(Y, tm.tm_year + 1900, not_a_year, is);
+                            checked_set(m, tm.tm_mon + 1, not_a_month, is);
+                            checked_set(d, tm.tm_mday, not_a_day, is);
+                            checked_set(H, tm.tm_hour, not_a_hour, is);
+                            checked_set(M, tm.tm_min, not_a_minute, is);
+                            checked_set(s, duration_cast<Duration>(seconds{tm.tm_sec}),
+                                        not_a_second, is);
+                        }
+                        is.setstate(err);
+#else
+                        // "%a %b %e %T %Y"
+                        auto nm = detail::weekday_names();
+                        auto i = detail::scan_keyword(is, nm.first, nm.second) - nm.first;
+                        checked_set(wd, static_cast<int>(i % 7), not_a_weekday, is);
+                        ws(is);
+                        nm = detail::month_names();
+                        i = detail::scan_keyword(is, nm.first, nm.second) - nm.first;
+                        checked_set(m, static_cast<int>(i % 12 + 1), not_a_month, is);
+                        ws(is);
+                        int td = not_a_day;
+                        read(is, rs{td, 1, 2});
+                        checked_set(d, td, not_a_day, is);
+                        ws(is);
+                        using dfs = detail::decimal_format_seconds<Duration>;
+                        CONSTDATA auto w = Duration::period::den == 1 ? 2 : 3 + dfs::width;
+                        int tH;
+                        int tM;
+                        long double S{};
+                        read(is, ru{tH, 1, 2}, CharT{':'}, ru{tM, 1, 2},
+                                               CharT{':'}, rld{S, 1, w});
+                        checked_set(H, tH, not_a_hour, is);
+                        checked_set(M, tM, not_a_minute, is);
+                        checked_set(s, round_i<Duration>(duration<long double>{S}),
+                                    not_a_second, is);
+                        ws(is);
+                        int tY = not_a_year;
+                        read(is, rs{tY, 1, 4u});
+                        checked_set(Y, tY, not_a_year, is);
+#endif
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'x':
+                if (command)
+                {
+                    if (modified != CharT{'O'})
+                    {
+#if !ONLY_C_LOCALE
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                        {
+                            checked_set(Y, tm.tm_year + 1900, not_a_year, is);
+                            checked_set(m, tm.tm_mon + 1, not_a_month, is);
+                            checked_set(d, tm.tm_mday, not_a_day, is);
+                        }
+                        is.setstate(err);
+#else
+                        // "%m/%d/%y"
+                        int ty = not_a_2digit_year;
+                        int tm = not_a_month;
+                        int td = not_a_day;
+                        read(is, ru{tm, 1, 2}, CharT{'/'}, ru{td, 1, 2}, CharT{'/'},
+                                 rs{ty, 1, 2});
+                        checked_set(y, ty, not_a_2digit_year, is);
+                        checked_set(m, tm, not_a_month, is);
+                        checked_set(d, td, not_a_day, is);
+#endif
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'X':
+                if (command)
+                {
+                    if (modified != CharT{'O'})
+                    {
+#if !ONLY_C_LOCALE
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                        {
+                            checked_set(H, tm.tm_hour, not_a_hour, is);
+                            checked_set(M, tm.tm_min, not_a_minute, is);
+                            checked_set(s, duration_cast<Duration>(seconds{tm.tm_sec}),
+                                        not_a_second, is);
+                        }
+                        is.setstate(err);
+#else
+                        // "%T"
+                        using dfs = detail::decimal_format_seconds<Duration>;
+                        CONSTDATA auto w = Duration::period::den == 1 ? 2 : 3 + dfs::width;
+                        int tH = not_a_hour;
+                        int tM = not_a_minute;
+                        long double S{};
+                        read(is, ru{tH, 1, 2}, CharT{':'}, ru{tM, 1, 2},
+                                               CharT{':'}, rld{S, 1, w});
+                        checked_set(H, tH, not_a_hour, is);
+                        checked_set(M, tM, not_a_minute, is);
+                        checked_set(s, round_i<Duration>(duration<long double>{S}),
+                                    not_a_second, is);
+#endif
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'C':
+                if (command)
+                {
+                    int tC = not_a_century;
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+                    {
+#endif
+                        read(is, rs{tC, 1, width == -1 ? 2u : static_cast<unsigned>(width)});
+#if !ONLY_C_LOCALE
+                    }
+                    else
+                    {
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                        {
+                            auto tY = tm.tm_year + 1900;
+                            tC = (tY >= 0 ? tY : tY-99) / 100;
+                        }
+                        is.setstate(err);
+                    }
+#endif
+                    checked_set(C, tC, not_a_century, is);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'D':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int tn = not_a_month;
+                        int td = not_a_day;
+                        int ty = not_a_2digit_year;
+                        read(is, ru{tn, 1, 2}, CharT{'\0'}, CharT{'/'}, CharT{'\0'},
+                                 ru{td, 1, 2}, CharT{'\0'}, CharT{'/'}, CharT{'\0'},
+                                 rs{ty, 1, 2});
+                        checked_set(y, ty, not_a_2digit_year, is);
+                        checked_set(m, tn, not_a_month, is);
+                        checked_set(d, td, not_a_day, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'F':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int tY = not_a_year;
+                        int tn = not_a_month;
+                        int td = not_a_day;
+                        read(is, rs{tY, 1, width == -1 ? 4u : static_cast<unsigned>(width)},
+                                 CharT{'-'}, ru{tn, 1, 2}, CharT{'-'}, ru{td, 1, 2});
+                        checked_set(Y, tY, not_a_year, is);
+                        checked_set(m, tn, not_a_month, is);
+                        checked_set(d, td, not_a_day, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'd':
+            case 'e':
+                if (command)
+                {
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#else
+                    if (modified != CharT{'E'})
+#endif
+                    {
+                        int td = not_a_day;
+                        read(is, rs{td, 1, width == -1 ? 2u : static_cast<unsigned>(width)});
+                        checked_set(d, td, not_a_day, is);
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        command = nullptr;
+                        width = -1;
+                        modified = CharT{};
+                        if ((err & ios::failbit) == 0)
+                            checked_set(d, tm.tm_mday, not_a_day, is);
+                        is.setstate(err);
+                    }
+#endif
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'H':
+                if (command)
+                {
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#else
+                    if (modified != CharT{'E'})
+#endif
+                    {
+                        int tH = not_a_hour;
+                        read(is, ru{tH, 1, width == -1 ? 2u : static_cast<unsigned>(width)});
+                        checked_set(H, tH, not_a_hour, is);
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                            checked_set(H, tm.tm_hour, not_a_hour, is);
+                        is.setstate(err);
+                    }
+#endif
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'I':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int tI = not_a_hour_12_value;
+                        // reads in an hour into I, but most be in [1, 12]
+                        read(is, rs{tI, 1, width == -1 ? 2u : static_cast<unsigned>(width)});
+                        if (!(1 <= tI && tI <= 12))
+                            is.setstate(ios::failbit);
+                        checked_set(I, tI, not_a_hour_12_value, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+               break;
+            case 'j':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int tj = not_a_doy;
+                        read(is, ru{tj, 1, width == -1 ? 3u : static_cast<unsigned>(width)});
+                        checked_set(j, tj, not_a_doy, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'M':
+                if (command)
+                {
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#else
+                    if (modified != CharT{'E'})
+#endif
+                    {
+                        int tM = not_a_minute;
+                        read(is, ru{tM, 1, width == -1 ? 2u : static_cast<unsigned>(width)});
+                        checked_set(M, tM, not_a_minute, is);
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                            checked_set(M, tm.tm_min, not_a_minute, is);
+                        is.setstate(err);
+                    }
+#endif
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'm':
+                if (command)
+                {
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#else
+                    if (modified != CharT{'E'})
+#endif
+                    {
+                        int tn = not_a_month;
+                        read(is, rs{tn, 1, width == -1 ? 2u : static_cast<unsigned>(width)});
+                        checked_set(m, tn, not_a_month, is);
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                            checked_set(m, tm.tm_mon + 1, not_a_month, is);
+                        is.setstate(err);
+                    }
+#endif
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'n':
+            case 't':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        // %n matches a single white space character
+                        // %t matches 0 or 1 white space characters
+                        auto ic = is.peek();
+                        if (Traits::eq_int_type(ic, Traits::eof()))
+                        {
+                            ios::iostate err = ios::eofbit;
+                            if (*fmt == 'n')
+                                err |= ios::failbit;
+                            is.setstate(err);
+                            break;
+                        }
+                        if (isspace(ic))
+                        {
+                            (void)is.get();
+                        }
+                        else if (*fmt == 'n')
+                            is.setstate(ios::failbit);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'p':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int tp = not_a_ampm;
+#if !ONLY_C_LOCALE
+                        tm = std::tm{};
+                        tm.tm_hour = 1;
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        is.setstate(err);
+                        if (tm.tm_hour == 1)
+                            tp = 0;
+                        else if (tm.tm_hour == 13)
+                            tp = 1;
+                        else
+                            is.setstate(err);
+#else
+                        auto nm = detail::ampm_names();
+                        auto i = detail::scan_keyword(is, nm.first, nm.second) - nm.first;
+                        tp = static_cast<decltype(tp)>(i);
+#endif
+                        checked_set(p, tp, not_a_ampm, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+
+               break;
+            case 'r':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+#if !ONLY_C_LOCALE
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                        {
+                            checked_set(H, tm.tm_hour, not_a_hour, is);
+                            checked_set(M, tm.tm_min, not_a_hour, is);
+                            checked_set(s, duration_cast<Duration>(seconds{tm.tm_sec}),
+                                        not_a_second, is);
+                        }
+                        is.setstate(err);
+#else
+                        // "%I:%M:%S %p"
+                        using dfs = detail::decimal_format_seconds<Duration>;
+                        CONSTDATA auto w = Duration::period::den == 1 ? 2 : 3 + dfs::width;
+                        long double S{};
+                        int tI = not_a_hour_12_value;
+                        int tM = not_a_minute;
+                        read(is, ru{tI, 1, 2}, CharT{':'}, ru{tM, 1, 2},
+                                               CharT{':'}, rld{S, 1, w});
+                        checked_set(I, tI, not_a_hour_12_value, is);
+                        checked_set(M, tM, not_a_minute, is);
+                        checked_set(s, round_i<Duration>(duration<long double>{S}),
+                                    not_a_second, is);
+                        ws(is);
+                        auto nm = detail::ampm_names();
+                        auto i = detail::scan_keyword(is, nm.first, nm.second) - nm.first;
+                        checked_set(p, static_cast<int>(i), not_a_ampm, is);
+#endif
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'R':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int tH = not_a_hour;
+                        int tM = not_a_minute;
+                        read(is, ru{tH, 1, 2}, CharT{'\0'}, CharT{':'}, CharT{'\0'},
+                                 ru{tM, 1, 2}, CharT{'\0'});
+                        checked_set(H, tH, not_a_hour, is);
+                        checked_set(M, tM, not_a_minute, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'S':
+                if (command)
+                {
+ #if !ONLY_C_LOCALE
+                   if (modified == CharT{})
+#else
+                   if (modified != CharT{'E'})
+#endif
+                    {
+                        using dfs = detail::decimal_format_seconds<Duration>;
+                        CONSTDATA auto w = Duration::period::den == 1 ? 2 : 3 + dfs::width;
+                        long double S{};
+                        read(is, rld{S, 1, width == -1 ? w : static_cast<unsigned>(width)});
+                        checked_set(s, round_i<Duration>(duration<long double>{S}),
+                                    not_a_second, is);
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'O'})
+                    {
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                            checked_set(s, duration_cast<Duration>(seconds{tm.tm_sec}),
+                                        not_a_second, is);
+                        is.setstate(err);
+                    }
+#endif
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'T':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        using dfs = detail::decimal_format_seconds<Duration>;
+                        CONSTDATA auto w = Duration::period::den == 1 ? 2 : 3 + dfs::width;
+                        int tH = not_a_hour;
+                        int tM = not_a_minute;
+                        long double S{};
+                        read(is, ru{tH, 1, 2}, CharT{':'}, ru{tM, 1, 2},
+                                               CharT{':'}, rld{S, 1, w});
+                        checked_set(H, tH, not_a_hour, is);
+                        checked_set(M, tM, not_a_minute, is);
+                        checked_set(s, round_i<Duration>(duration<long double>{S}),
+                                    not_a_second, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'Y':
+                if (command)
+                {
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#else
+                    if (modified != CharT{'O'})
+#endif
+                    {
+                        int tY = not_a_year;
+                        read(is, rs{tY, 1, width == -1 ? 4u : static_cast<unsigned>(width)});
+                        checked_set(Y, tY, not_a_year, is);
+                    }
+#if !ONLY_C_LOCALE
+                    else if (modified == CharT{'E'})
+                    {
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                            checked_set(Y, tm.tm_year + 1900, not_a_year, is);
+                        is.setstate(err);
+                    }
+#endif
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'y':
+                if (command)
+                {
+#if !ONLY_C_LOCALE
+                    if (modified == CharT{})
+#endif
+                    {
+                        int ty = not_a_2digit_year;
+                        read(is, ru{ty, 1, width == -1 ? 2u : static_cast<unsigned>(width)});
+                        checked_set(y, ty, not_a_2digit_year, is);
+                    }
+#if !ONLY_C_LOCALE
+                    else
+                    {
+                        ios::iostate err = ios::goodbit;
+                        f.get(is, nullptr, is, err, &tm, command, fmt+1);
+                        if ((err & ios::failbit) == 0)
+                            checked_set(Y, tm.tm_year + 1900, not_a_year, is);
+                        is.setstate(err);
+                    }
+#endif
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'g':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int tg = not_a_2digit_year;
+                        read(is, ru{tg, 1, width == -1 ? 2u : static_cast<unsigned>(width)});
+                        checked_set(g, tg, not_a_2digit_year, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'G':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int tG = not_a_year;
+                        read(is, rs{tG, 1, width == -1 ? 4u : static_cast<unsigned>(width)});
+                        checked_set(G, tG, not_a_year, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'U':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int tU = not_a_week_num;
+                        read(is, ru{tU, 1, width == -1 ? 2u : static_cast<unsigned>(width)});
+                        checked_set(U, tU, not_a_week_num, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'V':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int tV = not_a_week_num;
+                        read(is, ru{tV, 1, width == -1 ? 2u : static_cast<unsigned>(width)});
+                        checked_set(V, tV, not_a_week_num, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'W':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        int tW = not_a_week_num;
+                        read(is, ru{tW, 1, width == -1 ? 2u : static_cast<unsigned>(width)});
+                        checked_set(W, tW, not_a_week_num, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'E':
+            case 'O':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        modified = *fmt;
+                    }
+                    else
+                    {
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                        command = nullptr;
+                        width = -1;
+                        modified = CharT{};
+                    }
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case '%':
+                if (command)
+                {
+                    if (modified == CharT{})
+                        read(is, *fmt);
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    command = fmt;
+                break;
+            case 'z':
+                if (command)
+                {
+                    int tH, tM;
+                    minutes toff = not_a_offset;
+                    bool neg = false;
+                    auto ic = is.peek();
+                    if (!Traits::eq_int_type(ic, Traits::eof()))
+                    {
+                        auto c = static_cast<char>(Traits::to_char_type(ic));
+                        if (c == '-')
+                            neg = true;
+                    }
+                    if (modified == CharT{})
+                    {
+                        read(is, rs{tH, 2, 2});
+                        if (!is.fail())
+                            toff = hours{std::abs(tH)};
+                        if (is.good())
+                        {
+                            ic = is.peek();
+                            if (!Traits::eq_int_type(ic, Traits::eof()))
+                            {
+                                auto c = static_cast<char>(Traits::to_char_type(ic));
+                                if ('0' <= c && c <= '9')
+                                {
+                                    read(is, ru{tM, 2, 2});
+                                    if (!is.fail())
+                                        toff += minutes{tM};
+                                }
+                            }
+                        }
+                    }
+                    else
+                    {
+                        read(is, rs{tH, 1, 2});
+                        if (!is.fail())
+                            toff = hours{std::abs(tH)};
+                        if (is.good())
+                        {
+                            ic = is.peek();
+                            if (!Traits::eq_int_type(ic, Traits::eof()))
+                            {
+                                auto c = static_cast<char>(Traits::to_char_type(ic));
+                                if (c == ':')
+                                {
+                                    (void)is.get();
+                                    read(is, ru{tM, 2, 2});
+                                    if (!is.fail())
+                                        toff += minutes{tM};
+                                }
+                            }
+                        }
+                    }
+                    if (neg)
+                        toff = -toff;
+                    checked_set(temp_offset, toff, not_a_offset, is);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            case 'Z':
+                if (command)
+                {
+                    if (modified == CharT{})
+                    {
+                        std::basic_string<CharT, Traits, Alloc> buf;
+                        while (is.rdstate() == std::ios::goodbit)
+                        {
+                            auto i = is.rdbuf()->sgetc();
+                            if (Traits::eq_int_type(i, Traits::eof()))
+                            {
+                                is.setstate(ios::eofbit);
+                                break;
+                            }
+                            auto wc = Traits::to_char_type(i);
+                            auto c = static_cast<char>(wc);
+                            // is c a valid time zone name or abbreviation character?
+                            if (!(CharT{1} < wc && wc < CharT{127}) || !(isalnum(c) ||
+                                    c == '_' || c == '/' || c == '-' || c == '+'))
+                                break;
+                            buf.push_back(c);
+                            is.rdbuf()->sbumpc();
+                        }
+                        if (buf.empty())
+                            is.setstate(ios::failbit);
+                        checked_set(temp_abbrev, buf, {}, is);
+                    }
+                    else
+                        read(is, CharT{'%'}, width, modified, *fmt);
+                    command = nullptr;
+                    width = -1;
+                    modified = CharT{};
+                }
+                else
+                    read(is, *fmt);
+                break;
+            default:
+                if (command)
+                {
+                    if (width == -1 && modified == CharT{} && '0' <= *fmt && *fmt <= '9')
+                    {
+                        width = static_cast<char>(*fmt) - '0';
+                        while ('0' <= fmt[1] && fmt[1] <= '9')
+                            width = 10*width + static_cast<char>(*++fmt) - '0';
+                    }
+                    else
+                    {
+                        if (modified == CharT{})
+                            read(is, CharT{'%'}, width, *fmt);
+                        else
+                            read(is, CharT{'%'}, width, modified, *fmt);
+                        command = nullptr;
+                        width = -1;
+                        modified = CharT{};
+                    }
+                }
+                else  // !command
+                {
+                    if (isspace(static_cast<unsigned char>(*fmt)))
+                    {
+                        // space matches 0 or more white space characters
+                        if (is.good())
+                           ws(is);
+                    }
+                    else
+                        read(is, *fmt);
+                }
+                break;
+            }
+        }
+        // is.fail() || *fmt == CharT{}
+        if (is.rdstate() == ios::goodbit && command)
+        {
+            if (modified == CharT{})
+                read(is, CharT{'%'}, width);
+            else
+                read(is, CharT{'%'}, width, modified);
+        }
+        if (!is.fail())
+        {
+            if (y != not_a_2digit_year)
+            {
+                // Convert y and an optional C to Y
+                if (!(0 <= y && y <= 99))
+                    goto broken;
+                if (C == not_a_century)
+                {
+                    if (Y == not_a_year)
+                    {
+                        if (y >= 69)
+                            C = 19;
+                        else
+                            C = 20;
+                    }
+                    else
+                    {
+                        C = (Y >= 0 ? Y : Y-100) / 100;
+                    }
+                }
+                int tY;
+                if (C >= 0)
+                    tY = 100*C + y;
+                else
+                    tY = 100*(C+1) - (y == 0 ? 100 : y);
+                if (Y != not_a_year && Y != tY)
+                    goto broken;
+                Y = tY;
+            }
+            if (g != not_a_2digit_year)
+            {
+                // Convert g and an optional C to G
+                if (!(0 <= g && g <= 99))
+                    goto broken;
+                if (C == not_a_century)
+                {
+                    if (G == not_a_year)
+                    {
+                        if (g >= 69)
+                            C = 19;
+                        else
+                            C = 20;
+                    }
+                    else
+                    {
+                        C = (G >= 0 ? G : G-100) / 100;
+                    }
+                }
+                int tG;
+                if (C >= 0)
+                    tG = 100*C + g;
+                else
+                    tG = 100*(C+1) - (g == 0 ? 100 : g);
+                if (G != not_a_year && G != tG)
+                    goto broken;
+                G = tG;
+            }
+            if (Y < static_cast<int>(year::min()) || Y > static_cast<int>(year::max()))
+                Y = not_a_year;
+            bool computed = false;
+            if (G != not_a_year && V != not_a_week_num && wd != not_a_weekday)
+            {
+                year_month_day ymd_trial = sys_days(year{G-1}/December/Thursday[last]) +
+                                           (Monday-Thursday) + weeks{V-1} +
+                                           (weekday{static_cast<unsigned>(wd)}-Monday);
+                if (Y == not_a_year)
+                    Y = static_cast<int>(ymd_trial.year());
+                else if (year{Y} != ymd_trial.year())
+                    goto broken;
+                if (m == not_a_month)
+                    m = static_cast<int>(static_cast<unsigned>(ymd_trial.month()));
+                else if (month(static_cast<unsigned>(m)) != ymd_trial.month())
+                    goto broken;
+                if (d == not_a_day)
+                    d = static_cast<int>(static_cast<unsigned>(ymd_trial.day()));
+                else if (day(static_cast<unsigned>(d)) != ymd_trial.day())
+                    goto broken;
+                computed = true;
+            }
+            if (Y != not_a_year && U != not_a_week_num && wd != not_a_weekday)
+            {
+                year_month_day ymd_trial = sys_days(year{Y}/January/Sunday[1]) +
+                                           weeks{U-1} +
+                                           (weekday{static_cast<unsigned>(wd)} - Sunday);
+                if (Y == not_a_year)
+                    Y = static_cast<int>(ymd_trial.year());
+                else if (year{Y} != ymd_trial.year())
+                    goto broken;
+                if (m == not_a_month)
+                    m = static_cast<int>(static_cast<unsigned>(ymd_trial.month()));
+                else if (month(static_cast<unsigned>(m)) != ymd_trial.month())
+                    goto broken;
+                if (d == not_a_day)
+                    d = static_cast<int>(static_cast<unsigned>(ymd_trial.day()));
+                else if (day(static_cast<unsigned>(d)) != ymd_trial.day())
+                    goto broken;
+                computed = true;
+            }
+            if (Y != not_a_year && W != not_a_week_num && wd != not_a_weekday)
+            {
+                year_month_day ymd_trial = sys_days(year{Y}/January/Monday[1]) +
+                                           weeks{W-1} +
+                                           (weekday{static_cast<unsigned>(wd)} - Monday);
+                if (Y == not_a_year)
+                    Y = static_cast<int>(ymd_trial.year());
+                else if (year{Y} != ymd_trial.year())
+                    goto broken;
+                if (m == not_a_month)
+                    m = static_cast<int>(static_cast<unsigned>(ymd_trial.month()));
+                else if (month(static_cast<unsigned>(m)) != ymd_trial.month())
+                    goto broken;
+                if (d == not_a_day)
+                    d = static_cast<int>(static_cast<unsigned>(ymd_trial.day()));
+                else if (day(static_cast<unsigned>(d)) != ymd_trial.day())
+                    goto broken;
+                computed = true;
+            }
+            if (j != not_a_doy && Y != not_a_year)
+            {
+                auto ymd_trial = year_month_day{local_days(year{Y}/1/1) + days{j-1}};
+                if (m == not_a_month)
+                    m = static_cast<int>(static_cast<unsigned>(ymd_trial.month()));
+                else if (month(static_cast<unsigned>(m)) != ymd_trial.month())
+                    goto broken;
+                if (d == not_a_day)
+                    d = static_cast<int>(static_cast<unsigned>(ymd_trial.day()));
+                else if (day(static_cast<unsigned>(d)) != ymd_trial.day())
+                    goto broken;
+                j = not_a_doy;
+            }
+            auto ymd = year{Y}/m/d;
+            if (ymd.ok())
+            {
+                if (wd == not_a_weekday)
+                    wd = static_cast<int>((weekday(sys_days(ymd)) - Sunday).count());
+                else if (wd != static_cast<int>((weekday(sys_days(ymd)) - Sunday).count()))
+                    goto broken;
+                if (!computed)
+                {
+                    if (G != not_a_year || V != not_a_week_num)
+                    {
+                        sys_days sd = ymd;
+                        auto G_trial = year_month_day{sd + days{3}}.year();
+                        auto start = sys_days((G_trial - years{1})/December/Thursday[last]) +
+                                     (Monday - Thursday);
+                        if (sd < start)
+                        {
+                            --G_trial;
+                            if (V != not_a_week_num)
+                                start = sys_days((G_trial - years{1})/December/Thursday[last])
+                                        + (Monday - Thursday);
+                        }
+                        if (G != not_a_year && G != static_cast<int>(G_trial))
+                            goto broken;
+                        if (V != not_a_week_num)
+                        {
+                            auto V_trial = duration_cast<weeks>(sd - start).count() + 1;
+                            if (V != V_trial)
+                                goto broken;
+                        }
+                    }
+                    if (U != not_a_week_num)
+                    {
+                        auto start = sys_days(Sunday[1]/January/ymd.year());
+                        auto U_trial = floor<weeks>(sys_days(ymd) - start).count() + 1;
+                        if (U != U_trial)
+                            goto broken;
+                    }
+                    if (W != not_a_week_num)
+                    {
+                        auto start = sys_days(Monday[1]/January/ymd.year());
+                        auto W_trial = floor<weeks>(sys_days(ymd) - start).count() + 1;
+                        if (W != W_trial)
+                            goto broken;
+                    }
+                }
+            }
+            fds.ymd = ymd;
+            if (I != not_a_hour_12_value)
+            {
+                if (!(1 <= I && I <= 12))
+                    goto broken;
+                if (p != not_a_ampm)
+                {
+                    // p is in [0, 1] == [AM, PM]
+                    // Store trial H in I
+                    if (I == 12)
+                        --p;
+                    I += p*12;
+                    // Either set H from I or make sure H and I are consistent
+                    if (H == not_a_hour)
+                        H = I;
+                    else if (I != H)
+                        goto broken;
+                }
+                else  // p == not_a_ampm
+                {
+                    // if H, make sure H and I could be consistent
+                    if (H != not_a_hour)
+                    {
+                        if (I == 12)
+                        {
+                            if (H != 0 && H != 12)
+                                goto broken;
+                        }
+                        else if (!(I == H || I == H+12))
+                        {
+                            goto broken;
+                        }
+                    }
+                    else  // I is ambiguous, AM or PM?
+                        goto broken;
+                }
+            }
+            if (H != not_a_hour)
+            {
+                fds.has_tod = true;
+                fds.tod = hh_mm_ss<Duration>{hours{H}};
+            }
+            if (M != not_a_minute)
+            {
+                fds.has_tod = true;
+                fds.tod.m_ = minutes{M};
+            }
+            if (s != not_a_second)
+            {
+                fds.has_tod = true;
+                fds.tod.s_ = detail::decimal_format_seconds<Duration>{s};
+            }
+            if (j != not_a_doy)
+            {
+                fds.has_tod = true;
+                fds.tod.h_ += hours{days{j}};
+            }
+            if (wd != not_a_weekday)
+                fds.wd = weekday{static_cast<unsigned>(wd)};
+            if (abbrev != nullptr)
+                *abbrev = std::move(temp_abbrev);
+            if (offset != nullptr && temp_offset != not_a_offset)
+              *offset = temp_offset;
+        }
+       return is;
+    }
+broken:
+    is.setstate(ios::failbit);
+    return is;
+}
+
+template <class CharT, class Traits, class Alloc = std::allocator<CharT>>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt, year& y,
+            std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+            std::chrono::minutes* offset = nullptr)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{};
+    date::from_stream(is, fmt, fds, abbrev, offset);
+    if (!fds.ymd.year().ok())
+        is.setstate(std::ios::failbit);
+    if (!is.fail())
+        y = fds.ymd.year();
+    return is;
+}
+
+template <class CharT, class Traits, class Alloc = std::allocator<CharT>>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt, month& m,
+            std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+            std::chrono::minutes* offset = nullptr)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{};
+    date::from_stream(is, fmt, fds, abbrev, offset);
+    if (!fds.ymd.month().ok())
+        is.setstate(std::ios::failbit);
+    if (!is.fail())
+        m = fds.ymd.month();
+    return is;
+}
+
+template <class CharT, class Traits, class Alloc = std::allocator<CharT>>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt, day& d,
+            std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+            std::chrono::minutes* offset = nullptr)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{};
+    date::from_stream(is, fmt, fds, abbrev, offset);
+    if (!fds.ymd.day().ok())
+        is.setstate(std::ios::failbit);
+    if (!is.fail())
+        d = fds.ymd.day();
+    return is;
+}
+
+template <class CharT, class Traits, class Alloc = std::allocator<CharT>>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt, weekday& wd,
+            std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+            std::chrono::minutes* offset = nullptr)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{};
+    date::from_stream(is, fmt, fds, abbrev, offset);
+    if (!fds.wd.ok())
+        is.setstate(std::ios::failbit);
+    if (!is.fail())
+        wd = fds.wd;
+    return is;
+}
+
+template <class CharT, class Traits, class Alloc = std::allocator<CharT>>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt, year_month& ym,
+            std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+            std::chrono::minutes* offset = nullptr)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{};
+    date::from_stream(is, fmt, fds, abbrev, offset);
+    if (!fds.ymd.month().ok())
+        is.setstate(std::ios::failbit);
+    if (!is.fail())
+        ym = fds.ymd.year()/fds.ymd.month();
+    return is;
+}
+
+template <class CharT, class Traits, class Alloc = std::allocator<CharT>>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt, month_day& md,
+            std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+            std::chrono::minutes* offset = nullptr)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{};
+    date::from_stream(is, fmt, fds, abbrev, offset);
+    if (!fds.ymd.month().ok() || !fds.ymd.day().ok())
+        is.setstate(std::ios::failbit);
+    if (!is.fail())
+        md = fds.ymd.month()/fds.ymd.day();
+    return is;
+}
+
+template <class CharT, class Traits, class Alloc = std::allocator<CharT>>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt,
+            year_month_day& ymd, std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+            std::chrono::minutes* offset = nullptr)
+{
+    using CT = std::chrono::seconds;
+    fields<CT> fds{};
+    date::from_stream(is, fmt, fds, abbrev, offset);
+    if (!fds.ymd.ok())
+        is.setstate(std::ios::failbit);
+    if (!is.fail())
+        ymd = fds.ymd;
+    return is;
+}
+
+template <class Duration, class CharT, class Traits, class Alloc = std::allocator<CharT>>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt,
+            sys_time<Duration>& tp, std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+            std::chrono::minutes* offset = nullptr)
+{
+    using CT = typename std::common_type<Duration, std::chrono::seconds>::type;
+    using detail::round_i;
+    std::chrono::minutes offset_local{};
+    auto offptr = offset ? offset : &offset_local;
+    fields<CT> fds{};
+    fds.has_tod = true;
+    date::from_stream(is, fmt, fds, abbrev, offptr);
+    if (!fds.ymd.ok() || !fds.tod.in_conventional_range())
+        is.setstate(std::ios::failbit);
+    if (!is.fail())
+        tp = round_i<Duration>(sys_days(fds.ymd) - *offptr + fds.tod.to_duration());
+    return is;
+}
+
+template <class Duration, class CharT, class Traits, class Alloc = std::allocator<CharT>>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt,
+            local_time<Duration>& tp, std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+            std::chrono::minutes* offset = nullptr)
+{
+    using CT = typename std::common_type<Duration, std::chrono::seconds>::type;
+    using detail::round_i;
+    fields<CT> fds{};
+    fds.has_tod = true;
+    date::from_stream(is, fmt, fds, abbrev, offset);
+    if (!fds.ymd.ok() || !fds.tod.in_conventional_range())
+        is.setstate(std::ios::failbit);
+    if (!is.fail())
+        tp = round_i<Duration>(local_seconds{local_days(fds.ymd)} + fds.tod.to_duration());
+    return is;
+}
+
+template <class Rep, class Period, class CharT, class Traits, class Alloc = std::allocator<CharT>>
+std::basic_istream<CharT, Traits>&
+from_stream(std::basic_istream<CharT, Traits>& is, const CharT* fmt,
+            std::chrono::duration<Rep, Period>& d,
+            std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+            std::chrono::minutes* offset = nullptr)
+{
+    using Duration = std::chrono::duration<Rep, Period>;
+    using CT = typename std::common_type<Duration, std::chrono::seconds>::type;
+    using detail::round_i;
+    fields<CT> fds{};
+    date::from_stream(is, fmt, fds, abbrev, offset);
+    if (!fds.has_tod)
+        is.setstate(std::ios::failbit);
+    if (!is.fail())
+        d = round_i<Duration>(fds.tod.to_duration());
+    return is;
+}
+
+template <class Parsable, class CharT, class Traits = std::char_traits<CharT>,
+          class Alloc = std::allocator<CharT>>
+struct parse_manip
+{
+    const std::basic_string<CharT, Traits, Alloc> format_;
+    Parsable&                                     tp_;
+    std::basic_string<CharT, Traits, Alloc>*      abbrev_;
+    std::chrono::minutes*                         offset_;
+
+public:
+    parse_manip(std::basic_string<CharT, Traits, Alloc> format, Parsable& tp,
+                std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+                std::chrono::minutes* offset = nullptr)
+        : format_(std::move(format))
+        , tp_(tp)
+        , abbrev_(abbrev)
+        , offset_(offset)
+        {}
+
+#if HAS_STRING_VIEW
+    parse_manip(const CharT* format, Parsable& tp,
+                std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+                std::chrono::minutes* offset = nullptr)
+        : format_(format)
+        , tp_(tp)
+        , abbrev_(abbrev)
+        , offset_(offset)
+        {}
+
+    parse_manip(std::basic_string_view<CharT, Traits> format, Parsable& tp,
+                std::basic_string<CharT, Traits, Alloc>* abbrev = nullptr,
+                std::chrono::minutes* offset = nullptr)
+        : format_(format)
+        , tp_(tp)
+        , abbrev_(abbrev)
+        , offset_(offset)
+        {}
+#endif  // HAS_STRING_VIEW
+};
+
+template <class Parsable, class CharT, class Traits, class Alloc>
+std::basic_istream<CharT, Traits>&
+operator>>(std::basic_istream<CharT, Traits>& is,
+           const parse_manip<Parsable, CharT, Traits, Alloc>& x)
+{
+    return date::from_stream(is, x.format_.c_str(), x.tp_, x.abbrev_, x.offset_);
+}
+
+template <class Parsable, class CharT, class Traits, class Alloc>
+inline
+auto
+parse(const std::basic_string<CharT, Traits, Alloc>& format, Parsable& tp)
+    -> decltype(date::from_stream(std::declval<std::basic_istream<CharT, Traits>&>(),
+                            format.c_str(), tp),
+                parse_manip<Parsable, CharT, Traits, Alloc>{format, tp})
+{
+    return {format, tp};
+}
+
+template <class Parsable, class CharT, class Traits, class Alloc>
+inline
+auto
+parse(const std::basic_string<CharT, Traits, Alloc>& format, Parsable& tp,
+      std::basic_string<CharT, Traits, Alloc>& abbrev)
+    -> decltype(date::from_stream(std::declval<std::basic_istream<CharT, Traits>&>(),
+                            format.c_str(), tp, &abbrev),
+                parse_manip<Parsable, CharT, Traits, Alloc>{format, tp, &abbrev})
+{
+    return {format, tp, &abbrev};
+}
+
+template <class Parsable, class CharT, class Traits, class Alloc>
+inline
+auto
+parse(const std::basic_string<CharT, Traits, Alloc>& format, Parsable& tp,
+      std::chrono::minutes& offset)
+    -> decltype(date::from_stream(std::declval<std::basic_istream<CharT, Traits>&>(),
+                            format.c_str(), tp,
+                            std::declval<std::basic_string<CharT, Traits, Alloc>*>(),
+                            &offset),
+                parse_manip<Parsable, CharT, Traits, Alloc>{format, tp, nullptr, &offset})
+{
+    return {format, tp, nullptr, &offset};
+}
+
+template <class Parsable, class CharT, class Traits, class Alloc>
+inline
+auto
+parse(const std::basic_string<CharT, Traits, Alloc>& format, Parsable& tp,
+      std::basic_string<CharT, Traits, Alloc>& abbrev, std::chrono::minutes& offset)
+    -> decltype(date::from_stream(std::declval<std::basic_istream<CharT, Traits>&>(),
+                            format.c_str(), tp, &abbrev, &offset),
+                parse_manip<Parsable, CharT, Traits, Alloc>{format, tp, &abbrev, &offset})
+{
+    return {format, tp, &abbrev, &offset};
+}
+
+// const CharT* formats
+
+template <class Parsable, class CharT>
+inline
+auto
+parse(const CharT* format, Parsable& tp)
+    -> decltype(date::from_stream(std::declval<std::basic_istream<CharT>&>(), format, tp),
+                parse_manip<Parsable, CharT>{format, tp})
+{
+    return {format, tp};
+}
+
+template <class Parsable, class CharT, class Traits, class Alloc>
+inline
+auto
+parse(const CharT* format, Parsable& tp, std::basic_string<CharT, Traits, Alloc>& abbrev)
+    -> decltype(date::from_stream(std::declval<std::basic_istream<CharT, Traits>&>(), format,
+                            tp, &abbrev),
+                parse_manip<Parsable, CharT, Traits, Alloc>{format, tp, &abbrev})
+{
+    return {format, tp, &abbrev};
+}
+
+template <class Parsable, class CharT>
+inline
+auto
+parse(const CharT* format, Parsable& tp, std::chrono::minutes& offset)
+    -> decltype(date::from_stream(std::declval<std::basic_istream<CharT>&>(), format,
+                            tp, std::declval<std::basic_string<CharT>*>(), &offset),
+                parse_manip<Parsable, CharT>{format, tp, nullptr, &offset})
+{
+    return {format, tp, nullptr, &offset};
+}
+
+template <class Parsable, class CharT, class Traits, class Alloc>
+inline
+auto
+parse(const CharT* format, Parsable& tp,
+      std::basic_string<CharT, Traits, Alloc>& abbrev, std::chrono::minutes& offset)
+    -> decltype(date::from_stream(std::declval<std::basic_istream<CharT, Traits>&>(), format,
+                            tp, &abbrev, &offset),
+                parse_manip<Parsable, CharT, Traits, Alloc>{format, tp, &abbrev, &offset})
+{
+    return {format, tp, &abbrev, &offset};
+}
+
+// duration streaming
+
+template <class CharT, class Traits, class Rep, class Period>
+inline
+std::basic_ostream<CharT, Traits>&
+operator<<(std::basic_ostream<CharT, Traits>& os,
+           const std::chrono::duration<Rep, Period>& d)
+{
+    return os << detail::make_string<CharT, Traits>::from(d.count()) +
+                 detail::get_units<CharT>(typename Period::type{});
+}
+
+}  // namespace date
+
+#ifdef _MSC_VER
+#   pragma warning(pop)
+#endif
+
+#ifdef __GNUC__
+# pragma GCC diagnostic pop
+#endif
+
+// NOLINTEND
+// clang-format off
+
+#endif  // DATE_H
diff --git a/redfish-core/include/utils/time_utils.hpp b/redfish-core/include/utils/time_utils.hpp
index 314ea85de3..fbee6ce44e 100644
--- a/redfish-core/include/utils/time_utils.hpp
+++ b/redfish-core/include/utils/time_utils.hpp
@@ -2,8 +2,6 @@
 
 #include "logging.hpp"
 
-#include <boost/date_time.hpp>
-
 #include <algorithm>
 #include <charconv>
 #include <chrono>
@@ -420,30 +418,6 @@ inline std::pair<std::string, std::string> getDateTimeOffsetNow()
 }
 
 using usSinceEpoch = std::chrono::duration<uint64_t, std::micro>;
-inline std::optional<usSinceEpoch> dateStringToEpoch(std::string_view datetime)
-{
-    std::string date(datetime);
-    std::stringstream stream(date);
-    // Convert from ISO 8601 to boost local_time
-    // (BMC only has time in UTC)
-    boost::posix_time::ptime posixTime;
-    boost::posix_time::ptime epoch(boost::gregorian::date(1970, 1, 1));
-    // Facet gets deleted with the stringsteam
-    auto ifc = std::make_unique<boost::local_time::local_time_input_facet>(
-        "%Y-%m-%d %H:%M:%S%F %ZP");
-    stream.imbue(std::locale(stream.getloc(), ifc.release()));
-
-    boost::local_time::local_date_time ldt(boost::local_time::not_a_date_time);
-
-    if (!(stream >> ldt))
-    {
-        return std::nullopt;
-    }
-    posixTime = ldt.utc_time();
-    boost::posix_time::time_duration dur = posixTime - epoch;
-    uint64_t durMicroSecs = static_cast<uint64_t>(dur.total_microseconds());
-    return std::chrono::duration<uint64_t, std::micro>{durMicroSecs};
-}
-
+std::optional<usSinceEpoch> dateStringToEpoch(std::string_view datetime);
 } // namespace time_utils
 } // namespace redfish
diff --git a/redfish-core/src/utils/time_utils.cpp b/redfish-core/src/utils/time_utils.cpp
new file mode 100644
index 0000000000..f427ba64c4
--- /dev/null
+++ b/redfish-core/src/utils/time_utils.cpp
@@ -0,0 +1,44 @@
+#include "utils/time_utils.hpp"
+
+#include "utils/extern/date.h"
+
+#include <array>
+#include <chrono>
+#include <optional>
+#include <sstream>
+#include <string>
+#include <string_view>
+#include <version>
+
+namespace redfish::time_utils
+{
+using usSinceEpoch = std::chrono::duration<uint64_t, std::micro>;
+std::optional<usSinceEpoch> dateStringToEpoch(std::string_view datetime)
+{
+    for (const char* format : std::to_array({"%FT%T%Ez", "%FT%TZ", "%FT%T"}))
+    {
+        // Parse using signed so we can detect negative dates
+        std::chrono::sys_time<std::chrono::duration<int64_t, std::micro>> date;
+        std::istringstream iss(std::string{datetime});
+#if __cpp_lib_chrono >= 201907L
+        namespace chrono_from_stream = std::chrono;
+#else
+        namespace chrono_from_stream = date;
+#endif
+        if (chrono_from_stream::from_stream(iss, format, date))
+        {
+            if (date.time_since_epoch().count() < 0)
+            {
+                return std::nullopt;
+            }
+            if (iss.rdbuf()->in_avail() != 0)
+            {
+                // More information left at end of string.
+                continue;
+            }
+            return usSinceEpoch{date.time_since_epoch().count()};
+        }
+    }
+    return std::nullopt;
+}
+} // namespace redfish::time_utils
diff --git a/test/redfish-core/include/utils/time_utils_test.cpp b/test/redfish-core/include/utils/time_utils_test.cpp
index 873a385d1e..e88f7580d4 100644
--- a/test/redfish-core/include/utils/time_utils_test.cpp
+++ b/test/redfish-core/include/utils/time_utils_test.cpp
@@ -166,9 +166,8 @@ TEST(Utility, DateStringToEpoch)
               usSinceEpoch{253402300799999999});
 
     // Underflow
-    // Currently gives wrong result
-    // EXPECT_EQ(dateStringToEpoch("1969-12-30T23:59:59.999999+00:00"),
-    //          std::nullopt);
+    EXPECT_EQ(dateStringToEpoch("1969-12-30T23:59:59.999999+00:00"),
+              std::nullopt);
 }
 
 } // namespace