[libc++][chrono] Adds formatter file_time.

[Mordante]

Trigger CI.

[Mordante]

CI fixes.

[h-vetinari]

Is there anywhere where the missing <chrono> bits are tracked? I tracked down the "In progress" status for P0355 on https://libcxx.llvm.org/Status/Cxx20.html back to https://github.com/llvm/llvm-project/commit/ebff3123a2d7c6b3d74d27c50700f0582ec45e85, but that's a long time ago.

[Mordante]

CI fixes.

[Mordante]

In D148928#4289222, @h-vetinari wrote:

Is there anywhere where the missing <chrono> bits are tracked? I tracked down the "In progress" status for P0355 on https://libcxx.llvm.org/Status/Cxx20.html back to https://github.com/llvm/llvm-project/commit/ebff3123a2d7c6b3d74d27c50700f0582ec45e85, but that's a long time ago.

No not really. Some information is here https://libcxx.llvm.org/Status/Format.html (dependency "A <chrono> implementation"). I know there are no parse functions too. However I haven't spend time investigating the exact status. I still intend to work on chrono in the future, at which time I probably will create a status page. But for now I'm focusing on format and modules.

[h-vetinari]

In D148928#4289619, @Mordante wrote:

No not really. Some information is here https://libcxx.llvm.org/Status/Format.html (dependency "A <chrono> implementation"). I know there are no parse functions too. However I haven't spend time investigating the exact status. I still intend to work on chrono in the future, at which time I probably will create a status page. But for now I'm focusing on format and modules.

Not everything has to fall on you! But the combination of showing it "in progress" and not having a dedicated issue drastically lowers the visibility and thus the chance that someone would pick this up otherwise. Perhaps we should just open an issue?

I'm mainly curious about getting format out of experimental status, which at the time was blocked on chrono (well not exactly blocked, but you said you preferred to wait for chrono).

[Mordante]

In D148928#4290086, @h-vetinari wrote:

In D148928#4289619, @Mordante wrote:

No not really. Some information is here https://libcxx.llvm.org/Status/Format.html (dependency "A <chrono> implementation"). I know there are no parse functions too. However I haven't spend time investigating the exact status. I still intend to work on chrono in the future, at which time I probably will create a status page. But for now I'm focusing on format and modules.

Not everything has to fall on you!

True and I don't like it does. That something is on my todo list does not mean I will do it. Some items have been already been done before I had time to do them.

But the combination of showing it "in progress" and not having a dedicated issue drastically lowers the visibility and thus the chance that someone would pick this up otherwise. Perhaps we should just open an issue?

I don't feel GitHub issues helps us here. What would help is a status page that properly documents the status. But that is also quite some work since we don't know what exactly is done.

I'm mainly curious about getting format out of experimental status, which at the time was blocked on chrono (well not exactly blocked, but you said you preferred to wait for chrono).

I'm not sure at what time that exactly was. But I expect that was at a time I hadn't looked deeply at what chrono offers and what formatting challenges there are. At the moment I have a better understanding of chrono. I hope/expect that the experimental flag will be removed in LLVM 17. Note the feature-test macro does require the missing chrono parts to be available, so that will remain unset.

[H-G-Hristov]

@Mordante Sorry for the off-topic question. I am seen (I think) the same build issue with no report in all my builds from last week and retuning the CI doesn't fix them: "1145730 merge_guards_bot fail".
Am I doing something wrong or the CI isn't working currently?

[ldionne]

libcxx/test/std/time/time.syn/formatter.file_time.pass.cpp

44

[Mordante]

In D148928#4294797, @H-G-Hristov wrote:

@Mordante Sorry for the off-topic question. I am seen (I think) the same build issue with no report in all my builds from last week and retuning the CI doesn't fix them: "1145730 merge_guards_bot fail".
Am I doing something wrong or the CI isn't working currently?

According to Louis that is a recent change and the list of fixes should be looked at in buildkite itself. Note it would be better to ask these questions on Discord, that will result in faster replies.

libcxx/docs/Status/FormatPaper.csv

 Section,Description,Dependencies,Assignee,Status,First released version
 `P1361 <https://wg21.link/P1361>`__ `P2372 <https://wg21.link/P2372>`__,"Formatting chrono"
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::duration<Rep, Period>``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::sys_time<Duration>``",,Mark de Wever,|Complete|, Clang 17
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::utc_time<Duration>``",A ``<chrono>`` implementation,Not assigned,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::tai_time<Duration>``",A ``<chrono>`` implementation,Not assigned,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::gps_time<Duration>``",A ``<chrono>`` implementation,Not assigned,,,
-`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::file_time<Duration>``",,Mark de Wever,|In Progress|,,
+`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::file_time<Duration>``",,Mark de Wever,|Complete|, Clang 17
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::local_time<Duration>``",,Mark de Wever,|In Progress|,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::local-time-format-t<Duration>``",A ``<chrono>`` implementation,Not assigned,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::day``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::year``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::weekday``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::weekday_indexed``",,Mark de Wever,|Complete|, Clang 16
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::weekday_last``",,Mark de Wever,|Complete|, Clang 16

libcxx/include/__chrono/convert_to_tm.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___CHRONO_CONVERT_TO_TM_H
 #define _LIBCPP___CHRONO_CONVERT_TO_TM_H
 
 #include <__chrono/concepts.h>
 #include <__chrono/day.h>
 #include <__chrono/duration.h>
+#include <__chrono/file_clock.h>
 #include <__chrono/hh_mm_ss.h>
 #include <__chrono/month.h>
 #include <__chrono/month_weekday.h>
 #include <__chrono/monthday.h>
 #include <__chrono/statically_widen.h>
 #include <__chrono/system_clock.h>
 #include <__chrono/time_point.h>
 #include <__chrono/weekday.h>
#  endif
   __result.tm_yday =
       (static_cast<chrono::sys_days>(__date) -
        static_cast<chrono::sys_days>(chrono::year_month_day{__date.year(), chrono::January, chrono::day{1}}))
           .count();
 
   return __result;
 }
 
+template <class _Tm, class _Duration>
+_LIBCPP_HIDE_FROM_ABI _Tm __convert_to_tm(const chrono::sys_time<_Duration> __tp) {
+  chrono::sys_days __days = chrono::time_point_cast<chrono::days>(__tp);
+  chrono::year_month_day __ymd{__days};
+
+  _Tm __result = std::__convert_to_tm<_Tm>(chrono::year_month_day{__ymd}, chrono::weekday{__days});
+
+  uint64_t __sec =
+      chrono::duration_cast<chrono::seconds>(__tp - chrono::time_point_cast<chrono::seconds>(__days)).count();
+  __sec %= 24 * 3600;
+  __result.tm_hour = __sec / 3600;
+  __sec %= 3600;
+  __result.tm_min = __sec / 60;
+  __result.tm_sec = __sec % 60;
+
+  return __result;
+}
+
 // Convert a chrono (calendar) time point, or dururation to the given _Tm type,
 // which must have the same properties as std::tm.
 template <class _Tm, class _ChronoT>
 _LIBCPP_HIDE_FROM_ABI _Tm __convert_to_tm(const _ChronoT& __value) {
   _Tm __result = {};
 #  ifdef __GLIBC__
   __result.tm_zone = "UTC";
 #  endif
 
   if constexpr (__is_time_point<_ChronoT>) {
-    if constexpr (same_as<typename _ChronoT::clock, chrono::system_clock>) {
-      chrono::sys_days __days = chrono::time_point_cast<chrono::days>(__value);
-      chrono::year_month_day __ymd{__days};
-
-      __result = std::__convert_to_tm<_Tm>(chrono::year_month_day{__ymd}, chrono::weekday{__days});
-
-      // TODO FMT D138826 has improvements for this part.
-      // TODO FMT Since this is identical for duration and system time it would be good to avoid code duplication.
-      uint64_t __sec =
-          chrono::duration_cast<chrono::seconds>(__value - chrono::time_point_cast<chrono::seconds>(__days)).count();
-      __sec %= 24 * 3600;
-      __result.tm_hour = __sec / 3600;
-      __sec %= 3600;
-      __result.tm_min = __sec / 60;
-      __result.tm_sec = __sec % 60;
-    } else
+    if constexpr (same_as<typename _ChronoT::clock, chrono::system_clock>)
+      return std::__convert_to_tm<_Tm>(__value);
+    else if constexpr (same_as<typename _ChronoT::clock, chrono::file_clock>)
+      return std::__convert_to_tm<_Tm>(_ChronoT::clock::to_sys(__value));
+    else
       static_assert(sizeof(_ChronoT) == 0, "TODO: Add the missing clock specialization");
   } else if constexpr (chrono::__is_duration<_ChronoT>::value) {
     // [time.format]/6
     //   ...  However, if a flag refers to a "time of day" (e.g. %H, %I, %p,
     //   etc.), then a specialization of duration is interpreted as the time of
     //   day elapsed since midnight.
     uint64_t __sec = chrono::duration_cast<chrono::seconds>(__value).count();
     __sec %= 24 * 3600;

libcxx/include/__chrono/formatter.h

 #ifndef _LIBCPP___CHRONO_FORMATTER_H
 #define _LIBCPP___CHRONO_FORMATTER_H
 
 #include <__chrono/calendar.h>
 #include <__chrono/concepts.h>
 #include <__chrono/convert_to_tm.h>
 #include <__chrono/day.h>
 #include <__chrono/duration.h>
+#include <__chrono/file_clock.h>
 #include <__chrono/hh_mm_ss.h>
 #include <__chrono/month.h>
 #include <__chrono/month_weekday.h>
 #include <__chrono/monthday.h>
 #include <__chrono/ostream.h>
 #include <__chrono/parser_std_format_spec.h>
 #include <__chrono/statically_widen.h>
 #include <__chrono/system_clock.h>
public:
   using _Base = __formatter_chrono<_CharT>;
 
   _LIBCPP_HIDE_FROM_ABI constexpr auto parse(basic_format_parse_context<_CharT>& __parse_ctx)
       -> decltype(__parse_ctx.begin()) {
     return _Base::__parse(__parse_ctx, __format_spec::__fields_chrono, __format_spec::__flags::__clock);
   }
 };
 
+template <class _Duration, __fmt_char_type _CharT>
+struct _LIBCPP_TEMPLATE_VIS formatter<chrono::file_time<_Duration>, _CharT> : public __formatter_chrono<_CharT> {
+public:
+  using _Base = __formatter_chrono<_CharT>;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto parse(basic_format_parse_context<_CharT>& __parse_ctx)
+      -> decltype(__parse_ctx.begin()) {
+    return _Base::__parse(__parse_ctx, __format_spec::__fields_chrono, __format_spec::__flags::__clock);
+  }
+};
+
 template <class _Rep, class _Period, __fmt_char_type _CharT>
 struct formatter<chrono::duration<_Rep, _Period>, _CharT> : public __formatter_chrono<_CharT> {
 public:
   using _Base = __formatter_chrono<_CharT>;
 
   _LIBCPP_HIDE_FROM_ABI constexpr auto parse(basic_format_parse_context<_CharT>& __parse_ctx)
       -> decltype(__parse_ctx.begin()) {
     // [time.format]/1

libcxx/include/__chrono/ostream.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___CHRONO_OSTREAM_H
 #define _LIBCPP___CHRONO_OSTREAM_H
 
 #include <__chrono/day.h>
 #include <__chrono/duration.h>
+#include <__chrono/file_clock.h>
 #include <__chrono/hh_mm_ss.h>
 #include <__chrono/month.h>
 #include <__chrono/month_weekday.h>
 #include <__chrono/monthday.h>
 #include <__chrono/statically_widen.h>
 #include <__chrono/system_clock.h>
 #include <__chrono/weekday.h>
 #include <__chrono/year.h>

 namespace chrono {
 
 template <class _CharT, class _Traits, class _Duration>
 _LIBCPP_HIDE_FROM_ABI basic_ostream<_CharT, _Traits>&
 operator<<(basic_ostream<_CharT, _Traits>& __os, const sys_time<_Duration> __tp) {
   return __os << std::format(__os.getloc(), _LIBCPP_STATICALLY_WIDEN(_CharT, "{:L%F %T}"), __tp);
 }
 
+template <class _CharT, class _Traits, class _Duration>
+_LIBCPP_HIDE_FROM_ABI basic_ostream<_CharT, _Traits>&
+operator<<(basic_ostream<_CharT, _Traits>& __os, const file_time<_Duration> __tp) {
+  return __os << std::format(__os.getloc(), _LIBCPP_STATICALLY_WIDEN(_CharT, "{:L%F %T}"), __tp);
+}
+
 // Depending on the type the return is a const _CharT* or a basic_string<_CharT>
 template <class _CharT, class _Period>
 _LIBCPP_HIDE_FROM_ABI auto __units_suffix() {
   // TODO FMT LWG issue the suffixes are always char and not STATICALLY-WIDEN'ed.
   if constexpr (same_as<typename _Period::type, atto>)
     return _LIBCPP_STATICALLY_WIDEN(_CharT, "as");
   else if constexpr (same_as<typename _Period::type, femto>)
     return _LIBCPP_STATICALLY_WIDEN(_CharT, "fs");

libcxx/test/std/time/time.clock/time.clock.file/ostream.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-localization
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// TODO FMT Investigate Windows issues.
+// UNSUPPORTED msvc, target={{.+}}-windows-gnu
+// TODO FMT It seems GCC uses too much memory in the CI and fails.
+// UNSUPPORTED: gcc-12
+
+// TODO FMT This test should not require std::to_chars(floating-point)
+// XFAIL: availability-fp_to_chars-missing
+
+// REQUIRES: locale.fr_FR.UTF-8
+// REQUIRES: locale.ja_JP.UTF-8
+
+// <chrono>
+
+// class system_clock;
+
+// template<class charT, class traits, class Duration>
+//   basic_ostream<charT, traits>&
+//     operator<<(basic_ostream<charT, traits>& os, const sys_time<Duration>& tp);
+
+#include <chrono>
+#include <cassert>
+#include <ratio>
+#include <sstream>
+
+#include "make_string.h"
+#include "platform_support.h" // locale name macros
+#include "test_macros.h"
+
+#define SV(S) MAKE_STRING_VIEW(CharT, S)
+
+// Modeled after the system clock's usings, this make adapating the
+// tests easier.
+template <class Duration>
+using file_time    = std::chrono::time_point<std::chrono::file_clock, Duration>;
+using file_seconds = file_time<std::chrono::seconds>;
+using file_days    = file_time<std::chrono::days>;
+
+template <class CharT, class Duration>
+static std::basic_string<CharT> stream_c_locale(file_time<Duration> time_point) {
+  std::basic_stringstream<CharT> sstr;
+  sstr << std::fixed << time_point;
+  return sstr.str();
+}
+
+template <class CharT, class Duration>
+static std::basic_string<CharT> stream_fr_FR_locale(file_time<Duration> time_point) {
+  std::basic_stringstream<CharT> sstr;
+  const std::locale locale(LOCALE_fr_FR_UTF_8);
+  sstr.imbue(locale);
+  sstr << std::fixed << time_point;
+  return sstr.str();
+}
+
+template <class CharT, class Duration>
+static std::basic_string<CharT> stream_ja_JP_locale(file_time<Duration> time_point) {
+  std::basic_stringstream<CharT> sstr;
+  const std::locale locale(LOCALE_ja_JP_UTF_8);
+  sstr.imbue(locale);
+  sstr << std::fixed << time_point;
+  return sstr.str();
+}
+
+template <class CharT>
+static void test_c() {
+  using namespace std::literals::chrono_literals;
+
+  assert(stream_c_locale<CharT>(file_time<std::chrono::nanoseconds>{946'688'523'123'456'789ns}) ==
+         SV("2000-01-01 01:02:03.123456789"));
+  assert(stream_c_locale<CharT>(file_time<std::chrono::microseconds>{946'688'523'123'456us}) ==
+         SV("2000-01-01 01:02:03.123456"));
+
+  assert(stream_c_locale<CharT>(file_time<std::chrono::milliseconds>{946'684'800'123ms}) ==
+         SV("2000-01-01 00:00:00.123"));
+  assert(stream_c_locale<CharT>(file_seconds{1'234'567'890s}) == SV("2009-02-13 23:31:30"));
+  assert(stream_c_locale<CharT>(file_time<std::chrono::minutes>{20'576'131min}) == SV("2009-02-13 23:31:00"));
+  assert(stream_c_locale<CharT>(file_time<std::chrono::hours>{342'935h}) == SV("2009-02-13 23:00:00"));
+  assert(stream_c_locale<CharT>(file_days{std::chrono::days{14'288}}) == SV("2009-02-13 00:00:00"));
+  assert(stream_c_locale<CharT>(file_time<std::chrono::weeks>{std::chrono::weeks{2041}}) == SV("2009-02-12 00:00:00"));
+
+  assert(stream_c_locale<CharT>(file_time<std::chrono::duration<signed char, std::ratio<2, 1>>>{
+             std::chrono::duration<signed char, std::ratio<2, 1>>{60}}) == SV("1970-01-01 00:02:00"));
+  assert(stream_c_locale<CharT>(file_time<std::chrono::duration<short, std::ratio<1, 2>>>{
+             std::chrono::duration<short, std::ratio<1, 2>>{3600}}) == SV("1970-01-01 00:30:00.0"));
+  assert(stream_c_locale<CharT>(file_time<std::chrono::duration<int, std::ratio<1, 4>>>{
+             std::chrono::duration<int, std::ratio<1, 4>>{3600}}) == SV("1970-01-01 00:15:00.00"));
+  assert(stream_c_locale<CharT>(file_time<std::chrono::duration<long, std::ratio<1, 10>>>{
+             std::chrono::duration<long, std::ratio<1, 10>>{36611}}) == SV("1970-01-01 01:01:01.1"));
+  assert(stream_c_locale<CharT>(file_time<std::chrono::duration<long long, std::ratio<1, 100>>>{
+             std::chrono::duration<long long, std::ratio<1, 100>>{12'345'678'9010}}) == SV("2009-02-13 23:31:30.10"));
+
+  assert(stream_c_locale<CharT>(file_time<std::chrono::duration<float, std::ratio<1, 1>>>{
+             std::chrono::duration<float, std::ratio<1, 1>>{123.456}}) == SV("1970-01-01 00:02:03"));
+  assert(stream_c_locale<CharT>(file_time<std::chrono::duration<double, std::ratio<1, 10>>>{
+             std::chrono::duration<double, std::ratio<1, 10>>{123.456}}) == SV("1970-01-01 00:00:12.3"));
+  assert(stream_c_locale<CharT>(file_time<std::chrono::duration<long double, std::ratio<1, 100>>>{
+             std::chrono::duration<long double, std::ratio<1, 100>>{123.456}}) == SV("1970-01-01 00:00:01.23"));
+}
+
+template <class CharT>
+static void test_fr_FR() {
+  using namespace std::literals::chrono_literals;
+
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::nanoseconds>{946'688'523'123'456'789ns}) ==
+         SV("2000-01-01 01:02:03,123456789"));
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::microseconds>{946'688'523'123'456us}) ==
+         SV("2000-01-01 01:02:03,123456"));
+
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::milliseconds>{946'684'800'123ms}) ==
+         SV("2000-01-01 00:00:00,123"));
+  assert(stream_fr_FR_locale<CharT>(file_seconds{1'234'567'890s}) == SV("2009-02-13 23:31:30"));
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::minutes>{20'576'131min}) == SV("2009-02-13 23:31:00"));
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::hours>{342'935h}) == SV("2009-02-13 23:00:00"));
+  assert(stream_fr_FR_locale<CharT>(file_days{std::chrono::days{14'288}}) == SV("2009-02-13 00:00:00"));
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::weeks>{std::chrono::weeks{2041}}) ==
+         SV("2009-02-12 00:00:00"));
+
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::duration<signed char, std::ratio<2, 1>>>{
+             std::chrono::duration<signed char, std::ratio<2, 1>>{60}}) == SV("1970-01-01 00:02:00"));
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::duration<short, std::ratio<1, 2>>>{
+             std::chrono::duration<short, std::ratio<1, 2>>{3600}}) == SV("1970-01-01 00:30:00,0"));
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::duration<int, std::ratio<1, 4>>>{
+             std::chrono::duration<int, std::ratio<1, 4>>{3600}}) == SV("1970-01-01 00:15:00,00"));
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::duration<long, std::ratio<1, 10>>>{
+             std::chrono::duration<long, std::ratio<1, 10>>{36611}}) == SV("1970-01-01 01:01:01,1"));
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::duration<long long, std::ratio<1, 100>>>{
+             std::chrono::duration<long long, std::ratio<1, 100>>{12'345'678'9010}}) == SV("2009-02-13 23:31:30,10"));
+
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::duration<float, std::ratio<1, 1>>>{
+             std::chrono::duration<float, std::ratio<1, 1>>{123.456}}) == SV("1970-01-01 00:02:03"));
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::duration<double, std::ratio<1, 10>>>{
+             std::chrono::duration<double, std::ratio<1, 10>>{123.456}}) == SV("1970-01-01 00:00:12,3"));
+  assert(stream_fr_FR_locale<CharT>(file_time<std::chrono::duration<long double, std::ratio<1, 100>>>{
+             std::chrono::duration<long double, std::ratio<1, 100>>{123.456}}) == SV("1970-01-01 00:00:01,23"));
+}
+
+template <class CharT>
+static void test_ja_JP() {
+  using namespace std::literals::chrono_literals;
+
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::nanoseconds>{946'688'523'123'456'789ns}) ==
+         SV("2000-01-01 01:02:03.123456789"));
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::microseconds>{946'688'523'123'456us}) ==
+         SV("2000-01-01 01:02:03.123456"));
+
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::milliseconds>{946'684'800'123ms}) ==
+         SV("2000-01-01 00:00:00.123"));
+  assert(stream_ja_JP_locale<CharT>(file_seconds{1'234'567'890s}) == SV("2009-02-13 23:31:30"));
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::minutes>{20'576'131min}) == SV("2009-02-13 23:31:00"));
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::hours>{342'935h}) == SV("2009-02-13 23:00:00"));
+  assert(stream_ja_JP_locale<CharT>(file_days{std::chrono::days{14'288}}) == SV("2009-02-13 00:00:00"));
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::weeks>{std::chrono::weeks{2041}}) ==
+         SV("2009-02-12 00:00:00"));
+
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::duration<signed char, std::ratio<2, 1>>>{
+             std::chrono::duration<signed char, std::ratio<2, 1>>{60}}) == SV("1970-01-01 00:02:00"));
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::duration<short, std::ratio<1, 2>>>{
+             std::chrono::duration<short, std::ratio<1, 2>>{3600}}) == SV("1970-01-01 00:30:00.0"));
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::duration<int, std::ratio<1, 4>>>{
+             std::chrono::duration<int, std::ratio<1, 4>>{3600}}) == SV("1970-01-01 00:15:00.00"));
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::duration<long, std::ratio<1, 10>>>{
+             std::chrono::duration<long, std::ratio<1, 10>>{36611}}) == SV("1970-01-01 01:01:01.1"));
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::duration<long long, std::ratio<1, 100>>>{
+             std::chrono::duration<long long, std::ratio<1, 100>>{12'345'678'9010}}) == SV("2009-02-13 23:31:30.10"));
+
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::duration<float, std::ratio<1, 1>>>{
+             std::chrono::duration<float, std::ratio<1, 1>>{123.456}}) == SV("1970-01-01 00:02:03"));
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::duration<double, std::ratio<1, 10>>>{
+             std::chrono::duration<double, std::ratio<1, 10>>{123.456}}) == SV("1970-01-01 00:00:12.3"));
+  assert(stream_ja_JP_locale<CharT>(file_time<std::chrono::duration<long double, std::ratio<1, 100>>>{
+             std::chrono::duration<long double, std::ratio<1, 100>>{123.456}}) == SV("1970-01-01 00:00:01.23"));
+}
+
+template <class CharT>
+static void test() {
+  test_c<CharT>();
+  test_fr_FR<CharT>();
+  test_ja_JP<CharT>();
+}
+
+int main(int, char**) {
+  test<char>();
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}

libcxx/test/std/time/time.syn/formatter.file_time.pass.cpp

+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-localization
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// XFAIL: LIBCXX-FREEBSD-FIXME
+
+// TODO FMT This test should not require std::to_chars(floating-point)
+// XFAIL: availability-fp_to_chars-missing
+
+// TODO FMT Investigate Windows issues.
+// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
+// TODO FMT It seems GCC uses too much memory in the CI and fails.
+// UNSUPPORTED: gcc-12
+
+// REQUIRES: locale.fr_FR.UTF-8
+// REQUIRES: locale.ja_JP.UTF-8
+
+// <chrono>
+//
+// template<class charT> struct formatter<chrono::file_time, charT>;
+
+#include <chrono>
+#include <format>
+
+#include <cassert>
+#include <concepts>
+#include <locale>
+#include <iostream>
+#include <type_traits>
+
+#include "formatter_tests.h"
+#include "make_string.h"
+#include "platform_support.h" // locale name macros
+#include "test_macros.h"
+
+// Modeled after the system clock's usings, this make adapting the
+// tests easier.
+template <class Duration>
+using file_time    = std::chrono::time_point<std::chrono::file_clock, Duration>;
+using file_seconds = file_time<std::chrono::seconds>;
+using file_days    = file_time<std::chrono::days>;
+
+template <class CharT>
+static void test_no_chrono_specs() {
+  using namespace std::literals::chrono_literals;
+
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output
+  check(SV("1970-01-01 00:00:00"), SV("{}"), file_seconds(0s));
+  check(SV("2000-01-01 00:00:00"), SV("{}"), file_seconds(946'684'800s));
+  check(SV("2000-01-01 01:02:03"), SV("{}"), file_seconds(946'688'523s));
+
+  check(SV("2000-01-01 01:02:03.123"), SV("{}"), file_time<std::chrono::milliseconds>(946'688'523'123ms));
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values_year() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt =
+      SV("{:%%C='%C'%t%%EC='%EC'%t%%y='%y'%t%%Oy='%Oy'%t%%Ey='%Ey'%t%%Y='%Y'%t%%EY='%EY'%n}");
+  constexpr std::basic_string_view<CharT> lfmt =
+      SV("{:L%%C='%C'%t%%EC='%EC'%t%%y='%y'%t%%Oy='%Oy'%t%%Ey='%Ey'%t%%Y='%Y'%t%%EY='%EY'%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check(SV("%C='19'\t%EC='19'\t%y='70'\t%Oy='70'\t%Ey='70'\t%Y='1970'\t%EY='1970'\n"),
+        fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%C='20'\t%EC='20'\t%y='09'\t%Oy='09'\t%Ey='09'\t%Y='2009'\t%EY='2009'\n"),
+        fmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+
+  // Use the global locale (fr_FR)
+  check(SV("%C='19'\t%EC='19'\t%y='70'\t%Oy='70'\t%Ey='70'\t%Y='1970'\t%EY='1970'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%C='20'\t%EC='20'\t%y='09'\t%Oy='09'\t%Ey='09'\t%Y='2009'\t%EY='2009'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+#if defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%C='19'\t%EC='19'\t%y='70'\t%Oy='70'\t%Ey='70'\t%Y='1970'\t%EY='1970'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%C='20'\t%EC='20'\t%y='09'\t%Oy='09'\t%Ey='09'\t%Y='2009'\t%EY='2009'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#else                                  // defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%C='19'\t%EC='昭和'\t%y='70'\t%Oy='七十'\t%Ey='45'\t%Y='1970'\t%EY='昭和45年'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%C='20'\t%EC='平成'\t%y='09'\t%Oy='九'\t%Ey='21'\t%Y='2009'\t%EY='平成21年'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#endif                                 // defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values_month() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt  = SV("{:%%b='%b'%t%%h='%h'%t%%B='%B'%t%%m='%m'%t%%Om='%Om'%n}");
+  constexpr std::basic_string_view<CharT> lfmt = SV("{:L%%b='%b'%t%%h='%h'%t%%B='%B'%t%%m='%m'%t%%Om='%Om'%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check(SV("%b='Jan'\t%h='Jan'\t%B='January'\t%m='01'\t%Om='01'\n"),
+        fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%b='May'\t%h='May'\t%B='May'\t%m='05'\t%Om='05'\n"),
+        fmt,
+        file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+
+  // Use the global locale (fr_FR)
+#if defined(__APPLE__)
+  check(SV("%b='jan'\t%h='jan'\t%B='janvier'\t%m='01'\t%Om='01'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+#else
+  check(SV("%b='janv.'\t%h='janv.'\t%B='janvier'\t%m='01'\t%Om='01'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+#endif
+
+  check(SV("%b='mai'\t%h='mai'\t%B='mai'\t%m='05'\t%Om='05'\n"),
+        lfmt,
+        file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+#ifdef _WIN32
+  check(loc,
+        SV("%b='1'\t%h='1'\t%B='1月'\t%m='01'\t%Om='01'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%b='5'\t%h='5'\t%B='5月'\t%m='05'\t%Om='05'\n"),
+        lfmt,
+        file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+#elif defined(_AIX)                    // _WIN32
+  check(loc,
+        SV("%b='1月'\t%h='1月'\t%B='1月'\t%m='01'\t%Om='01'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%b='5月'\t%h='5月'\t%B='5月'\t%m='05'\t%Om='05'\n"),
+        lfmt,
+        file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+#elif defined(__APPLE__)               // _WIN32
+  check(loc,
+        SV("%b=' 1'\t%h=' 1'\t%B='1月'\t%m='01'\t%Om='01'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%b=' 5'\t%h=' 5'\t%B='5月'\t%m='05'\t%Om='05'\n"),
+        lfmt,
+        file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+#else                                  // _WIN32
+  check(loc,
+        SV("%b=' 1月'\t%h=' 1月'\t%B='1月'\t%m='01'\t%Om='一'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%b=' 5月'\t%h=' 5月'\t%B='5月'\t%m='05'\t%Om='五'\n"),
+        lfmt,
+        file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+#endif                                 // _WIN32
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values_day() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt  = SV("{:%%d='%d'%t%%Od='%Od'%t%%e='%e'%t%%Oe='%Oe'%n}");
+  constexpr std::basic_string_view<CharT> lfmt = SV("{:L%%d='%d'%t%%Od='%Od'%t%%e='%e'%t%%Oe='%Oe'%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check(SV("%d='01'\t%Od='01'\t%e=' 1'\t%Oe=' 1'\n"), fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%d='13'\t%Od='13'\t%e='13'\t%Oe='13'\n"),
+        fmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+
+  // Use the global locale (fr_FR)
+  check(SV("%d='01'\t%Od='01'\t%e=' 1'\t%Oe=' 1'\n"), lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%d='13'\t%Od='13'\t%e='13'\t%Oe='13'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+#if defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%d='01'\t%Od='01'\t%e=' 1'\t%Oe=' 1'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%d='13'\t%Od='13'\t%e='13'\t%Oe='13'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#else                                  // _WIN32
+  check(loc,
+        SV("%d='01'\t%Od='一'\t%e=' 1'\t%Oe='一'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%d='13'\t%Od='十三'\t%e='13'\t%Oe='十三'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+
+#endif                                 // _WIN32
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values_weekday() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt =
+      SV("{:%%a='%a'%t%%A='%A'%t%%u='%u'%t%%Ou='%Ou'%t%%w='%w'%t%%Ow='%Ow'%n}");
+  constexpr std::basic_string_view<CharT> lfmt =
+      SV("{:L%%a='%a'%t%%A='%A'%t%%u='%u'%t%%Ou='%Ou'%t%%w='%w'%t%%Ow='%Ow'%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check(SV("%a='Thu'\t%A='Thursday'\t%u='4'\t%Ou='4'\t%w='4'\t%Ow='4'\n"),
+        fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%a='Sun'\t%A='Sunday'\t%u='7'\t%Ou='7'\t%w='0'\t%Ow='0'\n"),
+        fmt,
+        file_seconds(4'294'967'295s)); // 06:28:15 UTC on Sunday, 7 February 2106
+
+  // Use the global locale (fr_FR)
+#if defined(__APPLE__)
+  check(SV("%a='Jeu'\t%A='Jeudi'\t%u='4'\t%Ou='4'\t%w='4'\t%Ow='4'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%a='Dim'\t%A='Dimanche'\t%u='7'\t%Ou='7'\t%w='0'\t%Ow='0'\n"),
+        lfmt,
+        file_seconds(4'294'967'295s)); // 06:28:15 UTC on Sunday, 7 February 2106
+#else
+  check(SV("%a='jeu.'\t%A='jeudi'\t%u='4'\t%Ou='4'\t%w='4'\t%Ow='4'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%a='dim.'\t%A='dimanche'\t%u='7'\t%Ou='7'\t%w='0'\t%Ow='0'\n"),
+        lfmt,
+        file_seconds(4'294'967'295s)); // 06:28:15 UTC on Sunday, 7 February 2106
+#endif
+
+  // Use supplied locale (ja_JP).
+  // This locale has a different alternate, but not on all platforms
+#if defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%a='木'\t%A='木曜日'\t%u='4'\t%Ou='4'\t%w='4'\t%Ow='4'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%a='日'\t%A='日曜日'\t%u='7'\t%Ou='7'\t%w='0'\t%Ow='0'\n"),
+        lfmt,
+        file_seconds(4'294'967'295s)); // 06:28:15 UTC on Sunday, 7 February 2106
+#else                                  // defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%a='木'\t%A='木曜日'\t%u='4'\t%Ou='四'\t%w='4'\t%Ow='四'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%a='日'\t%A='日曜日'\t%u='7'\t%Ou='七'\t%w='0'\t%Ow='〇'\n"),
+        lfmt,
+        file_seconds(4'294'967'295s)); // 06:28:15 UTC on Sunday, 7 February 2106
+#endif                                 // defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values_day_of_year() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt  = SV("{:%%j='%j'%n}");
+  constexpr std::basic_string_view<CharT> lfmt = SV("{:L%%j='%j'%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check(SV("%j='001'\n"), fmt, file_seconds(0s));             // 00:00:00 UTC Thursday, 1 January 1970
+  check(SV("%j='138'\n"), fmt, file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+
+  // Use the global locale (fr_FR)
+  check(SV("%j='001'\n"), lfmt, file_seconds(0s));             // 00:00:00 UTC Thursday, 1 January 1970
+  check(SV("%j='138'\n"), lfmt, file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+  check(loc, SV("%j='001'\n"), lfmt, file_seconds(0s));             // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc, SV("%j='138'\n"), lfmt, file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values_week() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt  = SV("{:%%U='%U'%t%%OU='%OU'%t%%W='%W'%t%%OW='%OW'%n}");
+  constexpr std::basic_string_view<CharT> lfmt = SV("{:L%%U='%U'%t%%OU='%OU'%t%%W='%W'%t%%OW='%OW'%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check(SV("%U='00'\t%OU='00'\t%W='00'\t%OW='00'\n"), fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%U='20'\t%OU='20'\t%W='20'\t%OW='20'\n"),
+        fmt,
+        file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+
+  // Use the global locale (fr_FR)
+  check(SV("%U='00'\t%OU='00'\t%W='00'\t%OW='00'\n"), lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%U='20'\t%OU='20'\t%W='20'\t%OW='20'\n"),
+        lfmt,
+        file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+#if defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%U='00'\t%OU='00'\t%W='00'\t%OW='00'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%U='20'\t%OU='20'\t%W='20'\t%OW='20'\n"),
+        lfmt,
+        file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+#else                                  // defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%U='00'\t%OU='〇'\t%W='00'\t%OW='〇'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%U='20'\t%OU='二十'\t%W='20'\t%OW='二十'\n"),
+        lfmt,
+        file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
+#endif                                 // defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values_iso_8601_week() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt  = SV("{:%%g='%g'%t%%G='%G'%t%%V='%V'%t%%OV='%OV'%n}");
+  constexpr std::basic_string_view<CharT> lfmt = SV("{:L%%g='%g'%t%%G='%G'%t%%V='%V'%t%%OV='%OV'%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check(SV("%g='70'\t%G='1970'\t%V='01'\t%OV='01'\n"), fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%g='09'\t%G='2009'\t%V='07'\t%OV='07'\n"),
+        fmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+
+  // Use the global locale (fr_FR)
+  check(SV("%g='70'\t%G='1970'\t%V='01'\t%OV='01'\n"), lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%g='09'\t%G='2009'\t%V='07'\t%OV='07'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+#if defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%g='70'\t%G='1970'\t%V='01'\t%OV='01'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%g='09'\t%G='2009'\t%V='07'\t%OV='07'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#else                                  // defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%g='70'\t%G='1970'\t%V='01'\t%OV='一'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%g='09'\t%G='2009'\t%V='07'\t%OV='七'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#endif                                 // defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values_date() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt  = SV("{:%%D='%D'%t%%F='%F'%t%%x='%x'%t%%Ex='%Ex'%n}");
+  constexpr std::basic_string_view<CharT> lfmt = SV("{:L%%D='%D'%t%%F='%F'%t%%x='%x'%t%%Ex='%Ex'%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check(SV("%D='01/01/70'\t%F='1970-01-01'\t%x='01/01/70'\t%Ex='01/01/70'\n"),
+        fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%D='02/13/09'\t%F='2009-02-13'\t%x='02/13/09'\t%Ex='02/13/09'\n"),
+        fmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+
+  // Use the global locale (fr_FR)
+#if defined(__APPLE__)
+  check(SV("%D='01/01/70'\t%F='1970-01-01'\t%x='01.01.1970'\t%Ex='01.01.1970'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%D='02/13/09'\t%F='2009-02-13'\t%x='13.02.2009'\t%Ex='13.02.2009'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#else
+  check(SV("%D='01/01/70'\t%F='1970-01-01'\t%x='01/01/1970'\t%Ex='01/01/1970'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%D='02/13/09'\t%F='2009-02-13'\t%x='13/02/2009'\t%Ex='13/02/2009'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#endif
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+#if defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%D='01/01/70'\t%F='1970-01-01'\t%x='1970/01/01'\t%Ex='1970/01/01'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%D='02/13/09'\t%F='2009-02-13'\t%x='2009/02/13'\t%Ex='2009/02/13'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#else                                  // defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%D='01/01/70'\t%F='1970-01-01'\t%x='1970年01月01日'\t%Ex='昭和45年01月01日'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%D='02/13/09'\t%F='2009-02-13'\t%x='2009年02月13日'\t%Ex='平成21年02月13日'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#endif                                 // defined(_WIN32) || defined(__APPLE__) || defined(_AIX)
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values_time() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt = SV(
+      "{:"
+      "%%H='%H'%t"
+      "%%OH='%OH'%t"
+      "%%I='%I'%t"
+      "%%OI='%OI'%t"
+      "%%M='%M'%t"
+      "%%OM='%OM'%t"
+      "%%S='%S'%t"
+      "%%OS='%OS'%t"
+      "%%p='%p'%t"
+      "%%R='%R'%t"
+      "%%T='%T'%t"
+      "%%r='%r'%t"
+      "%%X='%X'%t"
+      "%%EX='%EX'%t"
+      "%n}");
+  constexpr std::basic_string_view<CharT> lfmt = SV(
+      "{:L"
+      "%%H='%H'%t"
+      "%%OH='%OH'%t"
+      "%%I='%I'%t"
+      "%%OI='%OI'%t"
+      "%%M='%M'%t"
+      "%%OM='%OM'%t"
+      "%%S='%S'%t"
+      "%%OS='%OS'%t"
+      "%%p='%p'%t"
+      "%%R='%R'%t"
+      "%%T='%T'%t"
+      "%%r='%r'%t"
+      "%%X='%X'%t"
+      "%%EX='%EX'%t"
+      "%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check(SV("%H='00'\t"
+           "%OH='00'\t"
+           "%I='12'\t"
+           "%OI='12'\t"
+           "%M='00'\t"
+           "%OM='00'\t"
+           "%S='00'\t"
+           "%OS='00'\t"
+           "%p='AM'\t"
+           "%R='00:00'\t"
+           "%T='00:00:00'\t"
+           "%r='12:00:00 AM'\t"
+           "%X='00:00:00'\t"
+           "%EX='00:00:00'\t"
+           "\n"),
+        fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%H='23'\t"
+           "%OH='23'\t"
+           "%I='11'\t"
+           "%OI='11'\t"
+           "%M='31'\t"
+           "%OM='31'\t"
+           "%S='30.123'\t"
+           "%OS='30.123'\t"
+           "%p='PM'\t"
+           "%R='23:31'\t"
+           "%T='23:31:30.123'\t"
+           "%r='11:31:30 PM'\t"
+           "%X='23:31:30'\t"
+           "%EX='23:31:30'\t"
+           "\n"),
+        fmt,
+        std::chrono::sys_time<std::chrono::milliseconds>(
+            1'234'567'890'123ms)); // 23:31:30 UTC on Friday, 13 February 2009
+  // Use the global locale (fr_FR)
+  check(SV("%H='00'\t"
+           "%OH='00'\t"
+           "%I='12'\t"
+           "%OI='12'\t"
+           "%M='00'\t"
+           "%OM='00'\t"
+           "%S='00'\t"
+           "%OS='00'\t"
+#if defined(_AIX)
+           "%p='AM'\t"
+#else
+           "%p=''\t"
+#endif
+           "%R='00:00'\t"
+           "%T='00:00:00'\t"
+#ifdef _WIN32
+           "%r='00:00:00'\t"
+#elif defined(_AIX)
+           "%r='12:00:00 AM'\t"
+#elif defined(__APPLE__)
+           "%r=''\t"
+#else
+           "%r='12:00:00 '\t"
+#endif
+           "%X='00:00:00'\t"
+           "%EX='00:00:00'\t"
+           "\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%H='23'\t"
+           "%OH='23'\t"
+           "%I='11'\t"
+           "%OI='11'\t"
+           "%M='31'\t"
+           "%OM='31'\t"
+           "%S='30,123'\t"
+           "%OS='30,123'\t"
+#if defined(_AIX)
+           "%p='PM'\t"
+#else
+           "%p=''\t"
+#endif
+           "%R='23:31'\t"
+           "%T='23:31:30,123'\t"
+#ifdef _WIN32
+           "%r='23:31:30'\t"
+#elif defined(_AIX)
+           "%r='11:31:30 PM'\t"
+#elif defined(__APPLE__)
+           "%r=''\t"
+#else
+           "%r='11:31:30 '\t"
+#endif
+           "%X='23:31:30'\t"
+           "%EX='23:31:30'\t"
+           "\n"),
+        lfmt,
+        std::chrono::sys_time<std::chrono::milliseconds>(
+            1'234'567'890'123ms)); // 23:31:30 UTC on Friday, 13 February 2009
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.a
+#if defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%H='00'\t"
+           "%OH='00'\t"
+           "%I='12'\t"
+           "%OI='12'\t"
+           "%M='00'\t"
+           "%OM='00'\t"
+           "%S='00'\t"
+           "%OS='00'\t"
+#  if defined(__APPLE__)
+           "%p='AM'\t"
+#  else
+           "%p='午前'\t"
+#  endif
+           "%R='00:00'\t"
+           "%T='00:00:00'\t"
+#  if defined(__APPLE__)
+           "%r='12:00:00 AM'\t"
+           "%X='00時00分00秒'\t"
+           "%EX='00時00分00秒'\t"
+#  else
+           "%r='午前12:00:00'\t"
+           "%X='00:00:00'\t"
+           "%EX='00:00:00'\t"
+#  endif
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(0s));
+
+  check(loc,
+        SV("%H='23'\t"
+           "%OH='23'\t"
+           "%I='11'\t"
+           "%OI='11'\t"
+           "%M='31'\t"
+           "%OM='31'\t"
+           "%S='30.123'\t"
+           "%OS='30.123'\t"
+#  if defined(__APPLE__)
+           "%p='PM'\t"
+#  else
+           "%p='午後'\t"
+#  endif
+           "%R='23:31'\t"
+           "%T='23:31:30.123'\t"
+#  if defined(__APPLE__)
+           "%r='11:31:30 PM'\t"
+           "%X='23時31分30秒'\t"
+           "%EX='23時31分30秒'\t"
+#  else
+           "%r='午後11:31:30'\t"
+           "%X='23:31:30'\t"
+           "%EX='23:31:30'\t"
+#  endif
+           "\n"),
+        lfmt,
+        std::chrono::hh_mm_ss(23h + 31min + 30s + 123ms));
+#else  // defined(__APPLE__) || defined(_AIX)
+  check(loc,
+        SV("%H='00'\t"
+           "%OH='〇'\t"
+           "%I='12'\t"
+           "%OI='十二'\t"
+           "%M='00'\t"
+           "%OM='〇'\t"
+           "%S='00'\t"
+           "%OS='〇'\t"
+           "%p='午前'\t"
+           "%R='00:00'\t"
+           "%T='00:00:00'\t"
+           "%r='午前12時00分00秒'\t"
+           "%X='00時00分00秒'\t"
+           "%EX='00時00分00秒'\t"
+           "\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%H='23'\t"
+           "%OH='二十三'\t"
+           "%I='11'\t"
+           "%OI='十一'\t"
+           "%M='31'\t"
+           "%OM='三十一'\t"
+           "%S='30.123'\t"
+           "%OS='三十.123'\t"
+           "%p='午後'\t"
+           "%R='23:31'\t"
+           "%T='23:31:30.123'\t"
+           "%r='午後11時31分30秒'\t"
+           "%X='23時31分30秒'\t"
+           "%EX='23時31分30秒'\t"
+           "\n"),
+        lfmt,
+        std::chrono::sys_time<std::chrono::milliseconds>(
+            1'234'567'890'123ms)); // 23:31:30 UTC on Friday, 13 February 2009
+#endif // defined(__APPLE__) || defined(_AIX)
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values_date_time() {
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt  = SV("{:%%c='%c'%t%%Ec='%Ec'%n}");
+  constexpr std::basic_string_view<CharT> lfmt = SV("{:L%%c='%c'%t%%Ec='%Ec'%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+  // Non localized output using C-locale
+  check(SV("%c='Thu Jan  1 00:00:00 1970'\t%Ec='Thu Jan  1 00:00:00 1970'\n"),
+        fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(SV("%c='Fri Feb 13 23:31:30 2009'\t%Ec='Fri Feb 13 23:31:30 2009'\n"),
+        fmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+
+  // Use the global locale (fr_FR)
+  check(
+// https://sourceware.org/bugzilla/show_bug.cgi?id=24054
+#if defined(__GLIBC__) && __GLIBC__ <= 2 && __GLIBC_MINOR__ < 29
+      SV("%c='jeu. 01 janv. 1970 00:00:00 GMT'\t%Ec='jeu. 01 janv. 1970 00:00:00 GMT'\n"),
+#elif defined(_AIX)
+      SV("%c=' 1 janvier 1970 à 00:00:00 UTC'\t%Ec=' 1 janvier 1970 à 00:00:00 UTC'\n"),
+#elif defined(__APPLE__)
+      SV("%c='Jeu  1 jan 00:00:00 1970'\t%Ec='Jeu  1 jan 00:00:00 1970'\n"),
+#else
+      SV("%c='jeu. 01 janv. 1970 00:00:00'\t%Ec='jeu. 01 janv. 1970 00:00:00'\n"),
+#endif
+      lfmt,
+      file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(
+// https://sourceware.org/bugzilla/show_bug.cgi?id=24054
+#if defined(__GLIBC__) && __GLIBC__ <= 2 && __GLIBC_MINOR__ < 29
+      SV("%c='ven. 13 févr. 2009 23:31:30 GMT'\t%Ec='ven. 13 févr. 2009 23:31:30 GMT'\n"),
+#elif defined(_AIX)
+      SV("%c='13 février 2009 à 23:31:30 UTC'\t%Ec='13 février 2009 à 23:31:30 UTC'\n"),
+#elif defined(__APPLE__)
+      SV("%c='Ven 13 fév 23:31:30 2009'\t%Ec='Ven 13 fév 23:31:30 2009'\n"),
+#else
+      SV("%c='ven. 13 févr. 2009 23:31:30'\t%Ec='ven. 13 févr. 2009 23:31:30'\n"),
+#endif
+      lfmt,
+      file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.a
+#if defined(__APPLE__)
+  check(loc,
+        SV("%c='木  1/ 1 00:00:00 1970'\t%Ec='木  1/ 1 00:00:00 1970'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+  check(loc,
+        SV("%c='金  2/13 23:31:30 2009'\t%Ec='金  2/13 23:31:30 2009'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#elif defined(_AIX)                    // __APPLE__
+  check(loc,
+        SV("%c='1970年01月 1日 00:00:00 UTC'\t%Ec='1970年01月 1日 00:00:00 UTC'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+  check(loc,
+        SV("%c='2009年02月13日 23:31:30 UTC'\t%Ec='2009年02月13日 23:31:30 UTC'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#else                                  // __APPLE__
+  check(loc,
+        SV("%c='1970年01月01日 00時00分00秒'\t%Ec='昭和45年01月01日 00時00分00秒'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(loc,
+        SV("%c='2009年02月13日 23時31分30秒'\t%Ec='平成21年02月13日 23時31分30秒'\n"),
+        lfmt,
+        file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#endif                                 // __APPLE__
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test_valid_values_time_zone() {
+// The Apple CI gives %z='-0700'  %Ez='-0700' %Oz='-0700' %Z='UTC'
+// -0700 looks like the local time where the CI happens to recide, therefore
+// omit this test on Apple.
+#if !defined(__APPLE__)
+  using namespace std::literals::chrono_literals;
+
+  constexpr std::basic_string_view<CharT> fmt  = SV("{:%%z='%z'%t%%Ez='%Ez'%t%%Oz='%Oz'%t%%Z='%Z'%n}");
+  constexpr std::basic_string_view<CharT> lfmt = SV("{:L%%z='%z'%t%%Ez='%Ez'%t%%Oz='%Oz'%t%%Z='%Z'%n}");
+
+  const std::locale loc(LOCALE_ja_JP_UTF_8);
+  std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
+
+#  if defined(_AIX)
+  // Non localized output using C-locale
+  check(SV("%z='UTC'\t%Ez='UTC'\t%Oz='UTC'\t%Z='UTC'\n"),
+        fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  // Use the global locale (fr_FR)
+  check(SV("%z='UTC'\t%Ez='UTC'\t%Oz='UTC'\t%Z='UTC'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.a
+  check(loc,
+        SV("%z='UTC'\t%Ez='UTC'\t%Oz='UTC'\t%Z='UTC'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+#  else                    // defined(_AIX)
+  // Non localized output using C-locale
+  check(SV("%z='+0000'\t%Ez='+0000'\t%Oz='+0000'\t%Z='UTC'\n"),
+        fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  // Use the global locale (fr_FR)
+  check(SV("%z='+0000'\t%Ez='+0000'\t%Oz='+0000'\t%Z='UTC'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.a
+  check(loc,
+        SV("%z='+0000'\t%Ez='+0000'\t%Oz='+〇'\t%Z='UTC'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+#  endif                   // defined(_AIX)
+  std::locale::global(std::locale::classic());
+#endif                     // !defined(__APPLE__)
+}
+
+template <class CharT>
+static void test_valid_values() {
+  test_valid_values_year<CharT>();
+  test_valid_values_month<CharT>();
+  test_valid_values_day<CharT>();
+  test_valid_values_weekday<CharT>();
+  test_valid_values_day_of_year<CharT>();
+  test_valid_values_week<CharT>();
+  test_valid_values_iso_8601_week<CharT>();
+  test_valid_values_date<CharT>();
+  test_valid_values_time<CharT>();
+  test_valid_values_date_time<CharT>();
+  test_valid_values_time_zone<CharT>();
+}
+
+template <class CharT>
+static void test() {
+  using namespace std::literals::chrono_literals;
+
+  test_no_chrono_specs<CharT>();
+  test_valid_values<CharT>();
+
+  check_invalid_types<CharT>(
+      {SV("a"),  SV("A"),  SV("b"),  SV("B"),  SV("c"),  SV("C"),  SV("d"),  SV("D"),  SV("e"),  SV("F"),  SV("g"),
+       SV("G"),  SV("h"),  SV("H"),  SV("I"),  SV("j"),  SV("m"),  SV("M"),  SV("p"),  SV("r"),  SV("R"),  SV("S"),
+       SV("T"),  SV("u"),  SV("U"),  SV("V"),  SV("w"),  SV("W"),  SV("x"),  SV("X"),  SV("y"),  SV("Y"),  SV("z"),
+       SV("Z"),  SV("Ec"), SV("EC"), SV("Ex"), SV("EX"), SV("Ey"), SV("EY"), SV("Ez"), SV("Od"), SV("Oe"), SV("OH"),
+       SV("OI"), SV("Om"), SV("OM"), SV("OS"), SV("Ou"), SV("OU"), SV("OV"), SV("Ow"), SV("OW"), SV("Oy"), SV("Oz")},
+      file_seconds(0s));
+
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:A"), file_seconds(0s));
+  check_exception("The chrono-specs contains a '{'", SV("{:%%{"), file_seconds(0s));
+  check_exception("End of input while parsing the modifier chrono conversion-spec", SV("{:%"), file_seconds(0s));
+  check_exception("End of input while parsing the modifier E", SV("{:%E"), file_seconds(0s));
+  check_exception("End of input while parsing the modifier O", SV("{:%O"), file_seconds(0s));
+
+  // Precision not allowed
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:.3}"), file_seconds(0s));
+}
+
+int main(int, char**) {
+  test<char>();
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}

libcxx/test/std/utilities/format/format.formattable/concept.formattable.compile.pass.cpp

 #ifndef TEST_HAS_NO_LOCALIZATION
 
   assert_is_formattable<std::chrono::microseconds, CharT>();
 
   assert_is_formattable<std::chrono::sys_time<std::chrono::microseconds>, CharT>();
   //assert_is_formattable<std::chrono::utc_time<std::chrono::microseconds>, CharT>();
   //assert_is_formattable<std::chrono::tai_time<std::chrono::microseconds>, CharT>();
   //assert_is_formattable<std::chrono::gps_time<std::chrono::microseconds>, CharT>();
-  assert_is_not_formattable<std::chrono::file_time<std::chrono::microseconds>, CharT>();
+  assert_is_formattable<std::chrono::file_time<std::chrono::microseconds>, CharT>();
   assert_is_not_formattable<std::chrono::local_time<std::chrono::microseconds>, CharT>();
 
   assert_is_formattable<std::chrono::day, CharT>();
   assert_is_formattable<std::chrono::month, CharT>();
   assert_is_formattable<std::chrono::year, CharT>();
 
   assert_is_formattable<std::chrono::weekday, CharT>();
   assert_is_formattable<std::chrono::weekday_indexed, CharT>();