[libc++][chrono] Add sys_time formatter.

[Mordante]

Summary

Partially implements:

• P1361 Integration of chrono with text formatting
• P2372 Fixing locale handling in chrono formatters

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes

[Mordante]

CI fixes and temporary disable green CI jobs.

[Mordante]

CI fixes.

[Mordante]

CI fixes

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[Mordante]

CI fixes

[Mordante]

Rebased and enable the entire CI again.

[Mordante]

Disable Windows for now.

[Mordante]

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

21

Review all test to match the availablity as changed in D134598.

[ldionne]

libcxx/include/__chrono/convert_to_tm.h

102

It would be nice to implement this code path for the clocks that we do implement in libc++ for LLVM 17, to avoid the library being inconsistent with the clocks that it provides.

libcxx/include/__chrono/ostream.h

43

[Mordante]

Rebased and address review comments.

[Mordante]

libcxx/include/__chrono/convert_to_tm.h

102

I agree that would be nice. This is on my TODO list. Note that we still miss parts of chrono for the clocks and TZDB. Also the there is no support for parse. So in general C++20 chrono is not in a great shape in libc++. (Note all chrono things are quite high on my priority list, but not at the top. I hope to get some things in LLVM 17, everything seems unlikely due to how much time I have to work on libc++.)

[Mordante]

CI fixes

[Mordante]

CI fixes.

[Mordante]

CI fixes.

[bjope]

Started to see failures as shown below, downstream, after merging this patch.

So it seems like in my environment it says "GMT" instead of "UTC" (or perhaps the other way around).
Not sure exactly what might cause that and how to debug this problem. Any clues to what might cause this or what to look for?

FAIL: llvm-libc++-shared.cfg.in :: std/time/time.syn/formatter.sys_time.pass.cpp (79693 of 80953)
******************** TEST 'llvm-libc++-shared.cfg.in :: std/time/time.syn/formatter.sys_time.pass.cpp' FAILED ********************
Script:
--
: 'COMPILED WITH';  /repo/llvm-project/llvm/build-all-builtins/./bin/clang++ /repo/llvm-project/libcxx/test/std/time/time.syn/formatter.sys_time.pass.cpp  --target=x86_64-unknown-linux-gnu -nostdinc++ -I /repo/llvm-project/llvm/build-all-builtins/include/c++/v1 -I /repo/llvm-project/llvm/build-all-builtins/include/x86_64-unknown-linux-gnu/c++/v1 -I /repo/llvm-project/libcxx/test/support -std=c++2b -Werror -Wall -Wctad-maybe-unsupported -Wextra -Wshadow -Wundef -Wunused-template -Wno-unused-command-line-argument -Wno-attributes -Wno-pessimizing-move -Wno-c++11-extensions -Wno-noexcept-type -Wno-atomic-alignment -Wno-user-defined-literals -Wno-tautological-compare -Wsign-compare -Wunused-variable -Wunused-parameter -Wunreachable-code -Wno-unused-local-typedef -D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER -D_LIBCPP_ENABLE_EXPERIMENTAL -Werror=thread-safety -Wuser-defined-warnings  -lc++experimental -nostdlib++ -L /repo/llvm-project/llvm/build-all-builtins/./lib/x86_64-unknown-linux-gnu -Wl,-rpath,/repo/llvm-project/llvm/build-all-builtins/./lib/x86_64-unknown-linux-gnu -lc++ -pthread -o /repo/llvm-project/llvm/build-all-builtins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/test/std/time/time.syn/Output/formatter.sys_time.pass.cpp.dir/t.tmp.exe
: 'EXECUTED AS';  "/python/3.8.0/bin/python3.8" /repo/llvm-project/libcxx/test/../utils/run.py --execdir /repo/llvm-project/llvm/build-all-builtins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/test/std/time/time.syn/Output/formatter.sys_time.pass.cpp.dir --  /repo/llvm-project/llvm/build-all-builtins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/test/std/time/time.syn/Output/formatter.sys_time.pass.cpp.dir/t.tmp.exe
--
Exit Code: 250

Command Output (stdout):
--
$ ":" "COMPILED WITH"
note: command had no output on stdout or stderr
$ "/repo/llvm-project/llvm/build-all-builtins/./bin/clang++" "/repo/llvm-project/libcxx/test/std/time/time.syn/formatter.sys_time.pass.cpp" "--target=x86_64-unknown-linux-gnu" "-nostdinc++" "-I" "/repo/llvm-project/llvm/build-all-builtins/include/c++/v1" "-I" "/repo/llvm-project/llvm/build-all-builtins/include/x86_64-unknown-linux-gnu/c++/v1" "-I" "/repo/llvm-project/libcxx/test/support" "-std=c++2b" "-Werror" "-Wall" "-Wctad-maybe-unsupported" "-Wextra" "-Wshadow" "-Wundef" "-Wunused-template" "-Wno-unused-command-line-argument" "-Wno-attributes" "-Wno-pessimizing-move" "-Wno-c++11-extensions" "-Wno-noexcept-type" "-Wno-atomic-alignment" "-Wno-user-defined-literals" "-Wno-tautological-compare" "-Wsign-compare" "-Wunused-variable" "-Wunused-parameter" "-Wunreachable-code" "-Wno-unused-local-typedef" "-D_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER" "-D_LIBCPP_ENABLE_EXPERIMENTAL" "-Werror=thread-safety" "-Wuser-defined-warnings" "-lc++experimental" "-nostdlib++" "-L" "/repo/llvm-project/llvm/build-all-builtins/./lib/x86_64-unknown-linux-gnu" "-Wl,-rpath,/repo/llvm-project/llvm/build-all-builtins/./lib/x86_64-unknown-linux-gnu" "-lc++" "-pthread" "-o" "/repo/llvm-project/llvm/build-all-builtins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/test/std/time/time.syn/Output/formatter.sys_time.pass.cpp.dir/t.tmp.exe"
note: command had no output on stdout or stderr
$ ":" "EXECUTED AS"
note: command had no output on stdout or stderr
$ "/python/3.8.0/bin/python3.8" "/repo/llvm-project/libcxx/test/../utils/run.py" "--execdir" "/repo/llvm-project/llvm/build-all-builtins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/test/std/time/time.syn/Output/formatter.sys_time.pass.cpp.dir" "--" "/repo/llvm-project/llvm/build-all-builtins/runtimes/runtimes-x86_64-unknown-linux-gnu-bins/test/std/time/time.syn/Output/formatter.sys_time.pass.cpp.dir/t.tmp.exe"
# command stderr:

Format string   {:L%%c='%c'%t%%Ec='%Ec'%n}
Expected output %c='jeu. 01 janv. 1970 00:00:00 GMT'	%Ec='jeu. 01 janv. 1970 00:00:00 GMT'

Actual output   %c='jeu. 01 janv. 1970 00:00:00 UTC'	%Ec='jeu. 01 janv. 1970 00:00:00 UTC'

t.tmp.exe: /repo/llvm-project/libcxx/test/std/time/time.syn/formatter_tests.h :41 : void check(std::basic_string_view<CharT>, test_format_string<CharT, Args...>, Args &&...) [CharT = char, Args = <std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<long long>>>]:  l'assertion « out == expected » a échoué.

error: command failed with exit status: 250

--

********************
Testing:  0.. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90.. 

1 warning(s) in tests
********************
Failed Tests (1):
  llvm-libc++-shared.cfg.in :: std/time/time.syn/formatter.sys_time.pass.cpp

[Mordante]

In D144739#4260494, @bjope wrote:

Started to see failures as shown below, downstream, after merging this patch.

So it seems like in my environment it says "GMT" instead of "UTC" (or perhaps the other way around).
Not sure exactly what might cause that and how to debug this problem. Any clues to what might cause this or what to look for?

# command stderr:

Format string   {:L%%c='%c'%t%%Ec='%Ec'%n}
Expected output %c='jeu. 01 janv. 1970 00:00:00 GMT'	%Ec='jeu. 01 janv. 1970 00:00:00 GMT'

Actual output   %c='jeu. 01 janv. 1970 00:00:00 UTC'	%Ec='jeu. 01 janv. 1970 00:00:00 UTC'

t.tmp.exe: /repo/llvm-project/libcxx/test/std/time/time.syn/formatter_tests.h :41 : void check(std::basic_string_view<CharT>, test_format_string<CharT, Args...>, Args &&...) [CharT = char, Args = <std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<long long>>>]:  l'assertion « out == expected » a échoué.

error: command failed with exit status: 250

--

********************
Testing:  0.. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90.. 

1 warning(s) in tests
********************
Failed Tests (1):
  llvm-libc++-shared.cfg.in :: std/time/time.syn/formatter.sys_time.pass.cpp

These test highly depend on the platform specific libc's output. Which platform and libc are you using?
(Unfortunately it seems several platforms have other ideas how something is a specific locale should be.)

If you look at the failed test in this patch you already see quite some #ifdefs. The easiest solution for me would be to get a patch for your specific platform.

[bjope]

In D144739#4261870, @Mordante wrote:

In D144739#4260494, @bjope wrote:

Started to see failures as shown below, downstream, after merging this patch.

So it seems like in my environment it says "GMT" instead of "UTC" (or perhaps the other way around).
Not sure exactly what might cause that and how to debug this problem. Any clues to what might cause this or what to look for?

# command stderr:

Format string   {:L%%c='%c'%t%%Ec='%Ec'%n}
Expected output %c='jeu. 01 janv. 1970 00:00:00 GMT'	%Ec='jeu. 01 janv. 1970 00:00:00 GMT'

Actual output   %c='jeu. 01 janv. 1970 00:00:00 UTC'	%Ec='jeu. 01 janv. 1970 00:00:00 UTC'

t.tmp.exe: /repo/llvm-project/libcxx/test/std/time/time.syn/formatter_tests.h :41 : void check(std::basic_string_view<CharT>, test_format_string<CharT, Args...>, Args &&...) [CharT = char, Args = <std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<long long>>>]:  l'assertion « out == expected » a échoué.

error: command failed with exit status: 250

--

********************
Testing:  0.. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90.. 

1 warning(s) in tests
********************
Failed Tests (1):
  llvm-libc++-shared.cfg.in :: std/time/time.syn/formatter.sys_time.pass.cpp

These test highly depend on the platform specific libc's output. Which platform and libc are you using?
(Unfortunately it seems several platforms have other ideas how something is a specific locale should be.)

If you look at the failed test in this patch you already see quite some #ifdefs. The easiest solution for me would be to get a patch for your specific platform.

Right. I've noticed :-)

I'm on linux, RHEL7.9, targeting x86_64-unknown-linux-gnu. But not sure exactly what to look for. I just tried using a similar cmd line and adding "-dM", and it looks like I've got GLIBC 2.17. But no idea if this problem is specific to GLIBC or something else, so I wouldn't dare putting up a patch that expect UTC specifically for GLIBC 2.17.

For now I've just disabled the test case, so it is not causing me that much trouble. But I guess there might be others out there with similar problems.

[Mordante]

In D144739#4261996, @bjope wrote:

In D144739#4261870, @Mordante wrote:

In D144739#4260494, @bjope wrote:

Started to see failures as shown below, downstream, after merging this patch.

So it seems like in my environment it says "GMT" instead of "UTC" (or perhaps the other way around).
Not sure exactly what might cause that and how to debug this problem. Any clues to what might cause this or what to look for?

# command stderr:

Format string   {:L%%c='%c'%t%%Ec='%Ec'%n}
Expected output %c='jeu. 01 janv. 1970 00:00:00 GMT'	%Ec='jeu. 01 janv. 1970 00:00:00 GMT'

Actual output   %c='jeu. 01 janv. 1970 00:00:00 UTC'	%Ec='jeu. 01 janv. 1970 00:00:00 UTC'

t.tmp.exe: /repo/llvm-project/libcxx/test/std/time/time.syn/formatter_tests.h :41 : void check(std::basic_string_view<CharT>, test_format_string<CharT, Args...>, Args &&...) [CharT = char, Args = <std::chrono::time_point<std::chrono::system_clock, std::chrono::duration<long long>>>]:  l'assertion « out == expected » a échoué.

error: command failed with exit status: 250

--

********************
Testing:  0.. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90.. 

1 warning(s) in tests
********************
Failed Tests (1):
  llvm-libc++-shared.cfg.in :: std/time/time.syn/formatter.sys_time.pass.cpp

These test highly depend on the platform specific libc's output. Which platform and libc are you using?
(Unfortunately it seems several platforms have other ideas how something is a specific locale should be.)

If you look at the failed test in this patch you already see quite some #ifdefs. The easiest solution for me would be to get a patch for your specific platform.

Right. I've noticed :-)

I'm on linux, RHEL7.9, targeting x86_64-unknown-linux-gnu. But not sure exactly what to look for. I just tried using a similar cmd line and adding "-dM", and it looks like I've got GLIBC 2.17. But no idea if this problem is specific to GLIBC or something else, so I wouldn't dare putting up a patch that expect UTC specifically for GLIBC 2.17.

For now I've just disabled the test case, so it is not causing me that much trouble. But I guess there might be others out there with similar problems.

According to https://sourceware.org/glibc/wiki/Glibc%20Timeline GLIBC 2.17 is over 10 years old. We don't have a fixed policy on which versions of GLIBC we support. Since most users don't run our tests and you have solved it locally I prefer to keep the code as is. If there are more bug reports with newer GLIBC versions I can look at another #ifdef.

I'm actually surprised there are not more parts of the C library, we depend on, missing with such an old GLIBC version.

[philnik]

In D144739#4274408, @Mordante wrote:

According to https://sourceware.org/glibc/wiki/Glibc%20Timeline GLIBC 2.17 is over 10 years old. We don't have a fixed policy on which versions of GLIBC we support. Since most users don't run our tests and you have solved it locally I prefer to keep the code as is. If there are more bug reports with newer GLIBC versions I can look at another #ifdef.

I'm actually surprised there are not more parts of the C library, we depend on, missing with such an old GLIBC version.

We've actually started to only claim support for 2.24 and newer.

[Mordante]

In D144739#4274437, @philnik wrote:

In D144739#4274408, @Mordante wrote:

According to https://sourceware.org/glibc/wiki/Glibc%20Timeline GLIBC 2.17 is over 10 years old. We don't have a fixed policy on which versions of GLIBC we support. Since most users don't run our tests and you have solved it locally I prefer to keep the code as is. If there are more bug reports with newer GLIBC versions I can look at another #ifdef.

I'm actually surprised there are not more parts of the C library, we depend on, missing with such an old GLIBC version.

We've actually started to only claim support for 2.24 and newer.

Thanks it seems I missed that change. (I know we had a review about it, but missed we landed it.)

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,|In Progress|,
+`[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>``",,Not assigned,,,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::local_time<Duration>``",,Not assigned,,,
 `[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

libcxx/include/__chrono/concepts.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_CONCEPTS_H
 #define _LIBCPP___CHRONO_CONCEPTS_H
 
 #include <__chrono/hh_mm_ss.h>
+#include <__chrono/time_point.h>
 #include <__config>
 #include <__type_traits/is_specialization.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20
 
 template <class _Tp>
 concept __is_hh_mm_ss = __is_specialization_v<_Tp, chrono::hh_mm_ss>;
 
+template <class _Tp>
+concept __is_time_point = __is_specialization_v<_Tp, chrono::time_point>;
+
 #endif // _LIBCPP_STD_VER >= 20
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___CHRONO_CONCEPTS_H

libcxx/include/__chrono/convert_to_tm.h

 // 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 (chrono::__is_duration<_ChronoT>::value) {
+  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
+      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;
     __result.tm_hour = __sec / 3600;
     __sec %= 3600;

libcxx/include/__chrono/formatter.h

 #include <__chrono/duration.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>
 #include <__chrono/time_point.h>
 #include <__chrono/weekday.h>
 #include <__chrono/year.h>
 #include <__chrono/year_month.h>
 #include <__chrono/year_month_day.h>
 #include <__chrono/year_month_weekday.h>
 #include <__concepts/arithmetic.h>
 #include <__concepts/same_as.h>
std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
                    chrono::hh_mm_ss<__duration>::fractional_width);
   else
     std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
                    _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}}"),
                    __fraction.count(),
                    chrono::hh_mm_ss<__duration>::fractional_width);
 }
 
+template <class _CharT, __is_time_point _Tp>
+_LIBCPP_HIDE_FROM_ABI void __format_sub_seconds(const _Tp& __value, basic_stringstream<_CharT>& __sstr) {
+  __formatter::__format_sub_seconds(__value.time_since_epoch(), __sstr);
+}
+
 template <class _CharT, class _Duration>
 _LIBCPP_HIDE_FROM_ABI void
 __format_sub_seconds(const chrono::hh_mm_ss<_Duration>& __value, basic_stringstream<_CharT>& __sstr) {
   __sstr << std::use_facet<numpunct<_CharT>>(__sstr.getloc()).decimal_point();
   if constexpr (chrono::treat_as_floating_point_v<typename _Duration::rep>)
     std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
                    _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}.0f}"),
                    __value.subseconds().count(),
                    __value.fractional_width);
   else
     std::format_to(std::ostreambuf_iterator<_CharT>{__sstr},
                    _LIBCPP_STATICALLY_WIDEN(_CharT, "{:0{}}"),
                    __value.subseconds().count(),
                    __value.fractional_width);
 }
 
 template <class _Tp>
 consteval bool __use_fraction() {
-  if constexpr (chrono::__is_duration<_Tp>::value)
+  if constexpr (__is_time_point<_Tp>)
+    return chrono::hh_mm_ss<typename _Tp::duration>::fractional_width;
+  else if constexpr (chrono::__is_duration<_Tp>::value)
     return chrono::hh_mm_ss<_Tp>::fractional_width;
   else if constexpr (__is_hh_mm_ss<_Tp>)
     return _Tp::fractional_width;
   else
     return false;
 }
 
 template <class _CharT>
#  endif // defined(__GLIBC__) || defined(_AIX)
         int __year = __t.tm_year + 1900;
         if (__year < 1000) {
           __formatter::__format_year(__year, __sstr);
           __sstr << std::format(_LIBCPP_STATICALLY_WIDEN(_CharT, "-{:02}-{:02}"), __t.tm_mon + 1, __t.tm_mday);
         } else
           __facet.put({__sstr}, __sstr, _CharT(' '), std::addressof(__t), std::to_address(__s), std::to_address(__it + 1));
       } break;
 
+      case _CharT('Z'):
+        // TODO FMT Add proper timezone support.
+        __sstr << _LIBCPP_STATICALLY_WIDEN(_CharT, "UTC");
+        break;
+
       case _CharT('O'):
         if constexpr (__use_fraction<_Tp>()) {
           // Handle OS using the normal representation for the non-fractional
           // part. There seems to be no locale information regarding how the
           // fractional part should be formatted.
           if (*(__it + 1) == 'S') {
             ++__it;
             __facet.put({__sstr}, __sstr, _CharT(' '), std::addressof(__t), std::to_address(__s), std::to_address(__it + 1));
#  endif // defined(__GLIBC__) || defined(_AIX)
     } else {
       __sstr << *__it;
     }
   }
 }
 
 template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr bool __weekday_ok(const _Tp& __value) {
-  if constexpr (same_as<_Tp, chrono::day>)
+  if constexpr (__is_time_point<_Tp>)
+    return true;
+  else if constexpr (same_as<_Tp, chrono::day>)
     return true;
   else if constexpr (same_as<_Tp, chrono::month>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::year>)
     return true;
   else if constexpr (same_as<_Tp, chrono::weekday>)
     return true;
   else if constexpr (same_as<_Tp, chrono::weekday_indexed>)
_LIBCPP_HIDE_FROM_ABI constexpr bool __weekday_ok(const _Tp& __value) {
   else if constexpr (__is_hh_mm_ss<_Tp>)
     return true;
   else
     static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
 }
 
 template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr bool __weekday_name_ok(const _Tp& __value) {
-  if constexpr (same_as<_Tp, chrono::day>)
+  if constexpr (__is_time_point<_Tp>)
+    return true;
+  else if constexpr (same_as<_Tp, chrono::day>)
     return true;
   else if constexpr (same_as<_Tp, chrono::month>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::year>)
     return true;
   else if constexpr (same_as<_Tp, chrono::weekday>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::weekday_indexed>)
_LIBCPP_HIDE_FROM_ABI constexpr bool __weekday_name_ok(const _Tp& __value) {
   else if constexpr (__is_hh_mm_ss<_Tp>)
     return true;
   else
     static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
 }
 
 template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr bool __date_ok(const _Tp& __value) {
-  if constexpr (same_as<_Tp, chrono::day>)
+  if constexpr (__is_time_point<_Tp>)
+    return true;
+  else if constexpr (same_as<_Tp, chrono::day>)
     return true;
   else if constexpr (same_as<_Tp, chrono::month>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::year>)
     return true;
   else if constexpr (same_as<_Tp, chrono::weekday>)
     return true;
   else if constexpr (same_as<_Tp, chrono::weekday_indexed>)
_LIBCPP_HIDE_FROM_ABI constexpr bool __date_ok(const _Tp& __value) {
   else if constexpr (__is_hh_mm_ss<_Tp>)
     return true;
   else
     static_assert(sizeof(_Tp) == 0, "Add the missing type specialization");
 }
 
 template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr bool __month_name_ok(const _Tp& __value) {
-  if constexpr (same_as<_Tp, chrono::day>)
+  if constexpr (__is_time_point<_Tp>)
+    return true;
+  else if constexpr (same_as<_Tp, chrono::day>)
     return true;
   else if constexpr (same_as<_Tp, chrono::month>)
     return __value.ok();
   else if constexpr (same_as<_Tp, chrono::year>)
     return true;
   else if constexpr (same_as<_Tp, chrono::weekday>)
     return true;
   else if constexpr (same_as<_Tp, chrono::weekday_indexed>)
public:
   _LIBCPP_HIDE_FROM_ABI auto format(const _Tp& __value, auto& __ctx) const -> decltype(__ctx.out()) const {
     return __formatter::__format_chrono(
         __value, __ctx, __parser_.__parser_.__get_parsed_chrono_specifications(__ctx), __parser_.__chrono_specs_);
   }
 
   __format_spec::__parser_chrono<_CharT> __parser_;
 };
 
+template <class _Duration, __fmt_char_type _CharT>
+struct _LIBCPP_TEMPLATE_VIS formatter<chrono::sys_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

 
 #include <__chrono/day.h>
 #include <__chrono/duration.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>
 #include <__chrono/year_month.h>
 #include <__chrono/year_month_day.h>
 #include <__chrono/year_month_weekday.h>
 #include <__concepts/same_as.h>
 #include <__config>
 #include <__format/format_functions.h>
 #include <ostream>
 #include <ratio>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
 
 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);
+}
+
 // 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/include/chrono

public:
     static time_point from_time_t(time_t __t) noexcept;
 };
 
 template <class Duration>
   using sys_time  = time_point<system_clock, Duration>; // C++20
 using sys_seconds = sys_time<seconds>;                  // C++20
 using sys_days    = sys_time<days>;                     // C++20
 
+template<class charT, class traits, class Duration>     // C++20
+  basic_ostream<charT, traits>&
+    operator<<(basic_ostream<charT, traits>& os, const sys_time<Duration>& tp);
+
 class file_clock                                        // C++20
 {
 public:
     typedef see-below                      rep;
     typedef nano                           period;
     typedef chrono::duration<rep, period>  duration;
     typedef chrono::time_point<file_clock> time_point;
     static constexpr bool is_steady =      false;

 bool operator!=(const time_zone& x, const time_zone& y) noexcept;
 bool operator<(const time_zone& x, const time_zone& y) noexcept;
 bool operator>(const time_zone& x, const time_zone& y) noexcept;
 bool operator<=(const time_zone& x, const time_zone& y) noexcept;
 bool operator>=(const time_zone& x, const time_zone& y) noexcept;
 }  // chrono
 
 namespace std {
+  template<class Duration, class charT>
+    struct formatter<chrono::sys_time<Duration>, charT>;                          // C++20
   template<class Rep, class Period, class charT>
     struct formatter<chrono::duration<Rep, Period>, charT>;                       // C++20
   template<class charT> struct formatter<chrono::day, charT>;                     // C++20
   template<class charT> struct formatter<chrono::month, charT>;                   // C++20
   template<class charT> struct formatter<chrono::year, charT>;                    // C++20
   template<class charT> struct formatter<chrono::weekday, charT>;                 // C++20
   template<class charT> struct formatter<chrono::weekday_indexed, charT>;         // C++20
   template<class charT> struct formatter<chrono::weekday_last, charT>;            // C++20

libcxx/test/std/time/time.clock/time.clock.system/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)
+
+template <class CharT, class Duration>
+static std::basic_string<CharT> stream_c_locale(std::chrono::sys_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(std::chrono::sys_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(std::chrono::sys_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>(std::chrono::sys_time<std::chrono::nanoseconds>{946'688'523'123'456'789ns}) ==
+         SV("2000-01-01 01:02:03.123456789"));
+  assert(stream_c_locale<CharT>(std::chrono::sys_time<std::chrono::microseconds>{946'688'523'123'456us}) ==
+         SV("2000-01-01 01:02:03.123456"));
+
+  assert(stream_c_locale<CharT>(std::chrono::sys_time<std::chrono::milliseconds>{946'684'800'123ms}) ==
+         SV("2000-01-01 00:00:00.123"));
+  assert(stream_c_locale<CharT>(std::chrono::sys_seconds{1'234'567'890s}) == SV("2009-02-13 23:31:30"));
+  assert(stream_c_locale<CharT>(std::chrono::sys_time<std::chrono::minutes>{20'576'131min}) ==
+         SV("2009-02-13 23:31:00"));
+  assert(stream_c_locale<CharT>(std::chrono::sys_time<std::chrono::hours>{342'935h}) == SV("2009-02-13 23:00:00"));
+  assert(stream_c_locale<CharT>(std::chrono::sys_days{std::chrono::days{14'288}}) == SV("2009-02-13 00:00:00"));
+  assert(stream_c_locale<CharT>(std::chrono::sys_time<std::chrono::weeks>{std::chrono::weeks{2041}}) ==
+         SV("2009-02-12 00:00:00"));
+
+  assert(stream_c_locale<CharT>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_time<std::chrono::nanoseconds>{946'688'523'123'456'789ns}) ==
+         SV("2000-01-01 01:02:03,123456789"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::sys_time<std::chrono::microseconds>{946'688'523'123'456us}) ==
+         SV("2000-01-01 01:02:03,123456"));
+
+  assert(stream_fr_FR_locale<CharT>(std::chrono::sys_time<std::chrono::milliseconds>{946'684'800'123ms}) ==
+         SV("2000-01-01 00:00:00,123"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::sys_seconds{1'234'567'890s}) == SV("2009-02-13 23:31:30"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::sys_time<std::chrono::minutes>{20'576'131min}) ==
+         SV("2009-02-13 23:31:00"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::sys_time<std::chrono::hours>{342'935h}) == SV("2009-02-13 23:00:00"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::sys_days{std::chrono::days{14'288}}) == SV("2009-02-13 00:00:00"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::sys_time<std::chrono::weeks>{std::chrono::weeks{2041}}) ==
+         SV("2009-02-12 00:00:00"));
+
+  assert(stream_fr_FR_locale<CharT>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_time<std::chrono::nanoseconds>{946'688'523'123'456'789ns}) ==
+         SV("2000-01-01 01:02:03.123456789"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::sys_time<std::chrono::microseconds>{946'688'523'123'456us}) ==
+         SV("2000-01-01 01:02:03.123456"));
+
+  assert(stream_ja_JP_locale<CharT>(std::chrono::sys_time<std::chrono::milliseconds>{946'684'800'123ms}) ==
+         SV("2000-01-01 00:00:00.123"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::sys_seconds{1'234'567'890s}) == SV("2009-02-13 23:31:30"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::sys_time<std::chrono::minutes>{20'576'131min}) ==
+         SV("2009-02-13 23:31:00"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::sys_time<std::chrono::hours>{342'935h}) == SV("2009-02-13 23:00:00"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::sys_days{std::chrono::days{14'288}}) == SV("2009-02-13 00:00:00"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::sys_time<std::chrono::weeks>{std::chrono::weeks{2041}}) ==
+         SV("2009-02-12 00:00:00"));
+
+  assert(stream_ja_JP_locale<CharT>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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>(std::chrono::sys_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.sys_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::day, 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"
+
+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("{}"), std::chrono::sys_seconds(0s));
+  check(SV("2000-01-01 00:00:00"), SV("{}"), std::chrono::sys_seconds(946'684'800s));
+  check(SV("2000-01-01 01:02:03"), SV("{}"), std::chrono::sys_seconds(946'688'523s));
+
+  check(SV("2000-01-01 01:02:03.123"), SV("{}"), std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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, std::chrono::sys_seconds(0s));             // 00:00:00 UTC Thursday, 1 January 1970
+  check(SV("%j='138'\n"), fmt, std::chrono::sys_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, std::chrono::sys_seconds(0s));             // 00:00:00 UTC Thursday, 1 January 1970
+  check(SV("%j='138'\n"), lfmt, std::chrono::sys_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, std::chrono::sys_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  check(
+      loc, SV("%j='138'\n"), lfmt, std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+      std::chrono::sys_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,
+      std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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,
+        std::chrono::sys_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")},
+      std::chrono::sys_seconds(0s));
+
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:A"), std::chrono::sys_seconds(0s));
+  check_exception("The chrono-specs contains a '{'", SV("{:%%{"), std::chrono::sys_seconds(0s));
+  check_exception(
+      "End of input while parsing the modifier chrono conversion-spec", SV("{:%"), std::chrono::sys_seconds(0s));
+  check_exception("End of input while parsing the modifier E", SV("{:%E"), std::chrono::sys_seconds(0s));
+  check_exception("End of input while parsing the modifier O", SV("{:%O"), std::chrono::sys_seconds(0s));
+
+  // Precision not allowed
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:.3}"), std::chrono::sys_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

 //   locale imbued and copying os.str() through the output iterator of the
 //   context with additional padding and adjustments as specified by the format
 //   specifiers.
 // In libc++ std:::ostringstream requires localization support.
 #ifndef TEST_HAS_NO_LOCALIZATION
 
   assert_is_formattable<std::chrono::microseconds, CharT>();
 
-  assert_is_not_formattable<std::chrono::sys_time<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_not_formattable<std::chrono::local_time<std::chrono::microseconds>, CharT>();
 
   assert_is_formattable<std::chrono::day, CharT>();
   assert_is_formattable<std::chrono::month, CharT>();