[libc++][chrono] Implements formatter year.

[Mordante]

Summary

Partially implements:

• P1361 Integration of chrono with text formatting

[Mordante]

Attempts to fix CI.

[Mordante]

Fixes CI.

[Mordante]

Attempts to fix the CI.

[Mordante]

Fixing CI.

[Mordante]

Attempts to fix CI.

[Mordante]

Removes accidentally duplicated line.

[Mordante]

Attempts to fix CI.

[Mordante]

CI.

[Mordante]

Attempts to fix CI, only testing the affected ones.

[Mordante]

Readd removed CI runners.

[Mordante]

Avoids templates in tests.

[Mordante]

Rebased

[Mordante]

Trigger CI.

[Mordante]

Rebased to trigger CI.

[Mordante]

Fixes CI.

[Mordante]

Attempts to fix the CI.

[Mordante]

Fixes CI.

[Mordante]

CI fixes.

[Mordante]

CI fixes

[Mordante]

Attempts to fix CI.

[Mordante]

Polishing.

[ldionne]

LGTM.

libcxx/include/__chrono/formatter.h

61–63

Since this is used in a single place, any reason not to inline it where it's used?

68

Let's perform the tm => int-from-year-0 conversion in the caller of this function instead. Applies above and below too.

71

I don't understand this comment.

80

109–112

libcxx/test/libcxx/transitive_includes/cxx20.csv

79 ↗

(On Diff #462912)

This should go away once you rebase onto main with the updated transitive includes tests.

libcxx/test/std/time/time.cal/time.cal.year/time.cal.year.nonmembers/ostream.pass.cpp

11

Here and in the other tests.

[Mordante]

Thanks for the review!

libcxx/include/__chrono/formatter.h

61–63

As discussed I prefer to keep code in switch statements short. However since this is a one-liner I moved it.

[Mordante]

Rebased and addresses review comments.

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,|In Progress|,
 `[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::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,|In Progress|,
-`[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::year``",,Mark de Wever,|In Progress|,
+`[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,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::weekday_indexed``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::weekday_last``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month_day``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month_day_last``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month_weekday``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::month_weekday_last``",,Mark de Wever,|In Progress|,
 `[time.syn] <https://wg21.link/time.syn>`_,"Formatter ``chrono::year_month``",,Mark de Wever,|In Progress|,

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/day.h>
+#include <__chrono/year.h>
 #include <__concepts/same_as.h>
 #include <__config>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER > 17
 
 // Convert a chrono calendar time point to the given tm type,
 // which must have the same properties as std::tm.
 template <class _Tm, class _ChronoCalendarTimePoint>
 _LIBCPP_HIDE_FROM_ABI _Tm __convert_to_tm(const _ChronoCalendarTimePoint& __value) {
   _Tm __result = {};
 #  ifdef __GLIBC__
   __result.tm_zone = "UTC";
 #  endif
 
   if constexpr (same_as<_ChronoCalendarTimePoint, chrono::day>)
     __result.tm_mday = static_cast<unsigned>(__value);
+  else if constexpr (same_as<_ChronoCalendarTimePoint, chrono::year>)
+    __result.tm_year = static_cast<int>(__value) - 1900;
   else
     static_assert(sizeof(_ChronoCalendarTimePoint) == 0, "Add the missing type specialization");
 
   return __result;
 }
 
 #endif //if _LIBCPP_STD_VER > 17
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___CHRONO_CONVERT_TO_TM_H

libcxx/include/__chrono/formatter.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_FORMATTER_H
 #define _LIBCPP___CHRONO_FORMATTER_H
 
 #include <__chrono/convert_to_tm.h>
 #include <__chrono/day.h>
 #include <__chrono/parser_std_format_spec.h>
+#include <__chrono/statically_widen.h>
+#include <__chrono/year.h>
 #include <__config>
 #include <__format/concepts.h>
 #include <__format/format_parse_context.h>
 #include <__format/formatter.h>
 #include <__format/formatter_output.h>
 #include <__format/parser_std_format_spec.h>
+#include <__memory/addressof.h>
 #include <ctime>
+#include <format>
 #include <sstream>
 #include <string>
 #include <string_view>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 

 ///
 /// \pre When the (abbreviated) weekday or month name are used, the caller
 ///      validates whether the value is valid. So the caller handles that
 ///      requirement of Table 97: Meaning of conversion specifiers
 ///      [tab:time.format.spec].
 ///
 /// When no chrono-specs are provided it uses the stream formatter.
 
+template <class _CharT>
+_LIBCPP_HIDE_FROM_ABI void __format_year(int __year, basic_stringstream<_CharT>& __sstr) {
+  __year += 1900; // tm_year to year conversion.
+  if (__year < 0) {
+    __sstr << _CharT('-');
+    // The chrono library requires a year fits in a short.
+    __year = -__year;
+  }
+
+  __sstr << std::format(_LIBCPP_STATICALLY_WIDEN(_CharT, "{:04}"), __year);
+}
+
 template <class _CharT, class _Tp>
 _LIBCPP_HIDE_FROM_ABI void __format_chrono_using_chrono_specs(
     const _Tp& __value, basic_stringstream<_CharT>& __sstr, basic_string_view<_CharT> __chrono_specs) {
   tm __t              = std::__convert_to_tm<tm>(__value);
   const auto& __facet = std::use_facet<time_put<_CharT>>(__sstr.getloc());
   for (auto __it = __chrono_specs.begin(); __it != __chrono_specs.end(); ++__it) {
     if (*__it == _CharT('%')) {
       auto __s = __it;
       ++__it;
       // We only handle the types that can't be directly handled by time_put.
       // (as an optimization n, t, and % are also handled directly.)
       switch (*__it) {
       case _CharT('n'):
         __sstr << _CharT('\n');
         break;
       case _CharT('t'):
         __sstr << _CharT('\t');
         break;
       case _CharT('%'):
         __sstr << *__it;
         break;
 
+        // Unlike time_put and strftime the formatting library requires %Y
+        //
+        // [tab:time.format.spec]
+        //   The year as a decimal number. If the result is less than four digits
+        //   it is left-padded with 0 to four digits.
+        //
+        // This means years in the range (-1000, 1000) need manual formatting.
+        // It's unclear whether %EY needs the same treatment. For example the
+        // Japanese EY contains the era name and year. This is zero padded to 2
+        // digits time_put (note older glibc versions didn't do padding.)
+        // However most eras won't reach a 100 years, let alone 1000. So
+        // padding to 4 digits seems unwanted for Japanese.
+        //
+        // The same applies to %Ex since that too depends on the era.
+        //
+        // %x the locale's date representation is currently doesn't handle the
+        // zero-padding too.
+        //
+        // The 4 digits can be implemented better at a later time. On POSIX
+        // systems the required information can be extracted by nl_langinfo
+        // https://man7.org/linux/man-pages/man3/nl_langinfo.3.html
+        //
+        // Note since year < -1000 is expected to be rare it uses the more
+        // expensive year routine.
+        //
+        // TODO FMT evaluate the comment above.
+
+      case _CharT('Y'):
+        if (__t.tm_year < 1000)
+          __formatter::__format_year(__t.tm_year, __sstr);
+        else
+          __facet.put({__sstr}, __sstr, _CharT(' '), std::addressof(__t), __s, __it + 1);
+        break;
+
       case _CharT('O'):
+      case _CharT('E'):
         ++__it;
         [[fallthrough]];
       default:
         __facet.put({__sstr}, __sstr, _CharT(' '), std::addressof(__t), __s, __it + 1);
         break;
       }
     } else {
       __sstr << *__it;
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::__day);
   }
 };
 
-#endif //if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
+template <__fmt_char_type _CharT>
+struct _LIBCPP_TEMPLATE_VIS _LIBCPP_AVAILABILITY_FORMAT formatter<chrono::year, _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::__year);
+  }
+};
+
+#endif // if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_LOCALIZATION)
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif //  _LIBCPP___CHRONO_FORMATTER_H

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/statically_widen.h>
+#include <__chrono/year.h>
 #include <__config>
 #include <format>
 #include <ostream>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
return __os
               // Note this error differs from the wording of the Standard. The
               // Standard wording doesn't work well on AIX or Windows. There
               // the formatted day seems to be either modulo 100 or completely
               // omitted. Judging by the wording this is valid.
               // TODO FMT Write a paper of file an LWG issue.
               : std::format(_LIBCPP_STATICALLY_WIDEN(_CharT, "{:02} is not a valid day"), static_cast<unsigned>(__d)));
 }
 
+template <class _CharT, class _Traits>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_AVAILABILITY_FORMAT basic_ostream<_CharT, _Traits>&
+operator<<(basic_ostream<_CharT, _Traits>& __os, const year& __y) {
+  return __os << (__y.ok() ? std::format(_LIBCPP_STATICALLY_WIDEN(_CharT, "{:%Y}"), __y)
+                           : std::format(_LIBCPP_STATICALLY_WIDEN(_CharT, "{:%Y} is not a valid year"), __y));
+}
+
 } // namespace chrono
 
 #endif //if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___CHRONO_OSTREAM_H

libcxx/include/chrono

 class year;
 constexpr bool operator==(const year& x, const year& y) noexcept;
 constexpr strong_ordering operator<=>(const year& x, const year& y) noexcept;
 
 constexpr year  operator+(const year&  x, const years& y) noexcept;
 constexpr year  operator+(const years& x, const year&  y) noexcept;
 constexpr year  operator-(const year&  x, const years& y) noexcept;
 constexpr years operator-(const year&  x, const year&  y) noexcept;
+template<class charT, class traits>
+  basic_ostream<charT, traits>&
+    operator<<(basic_ostream<charT, traits>& os, const year& y);
 
 // 25.8.6, class weekday    // C++20
 class weekday;
 
 constexpr bool operator==(const weekday& x, const weekday& y) noexcept;
 constexpr bool operator!=(const weekday& x, const weekday& y) noexcept;
 constexpr weekday operator+(const weekday& x, const days&    y) noexcept;
 constexpr weekday operator+(const days&    x, const weekday& 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 charT> struct formatter<chrono::day, charT>;                     // C++20
+  template<class charT> struct formatter<chrono::year, charT>;                    // C++20
 } // namespace std
 
 namespace chrono {
 // calendrical constants
 inline constexpr last_spec                              last{};       // C++20
 inline constexpr chrono::weekday                        Sunday{0};    // C++20
 inline constexpr chrono::weekday                        Monday{1};    // C++20
 inline constexpr chrono::weekday                        Tuesday{2};   // C++20

libcxx/test/std/time/time.cal/time.cal.year/time.cal.year.nonmembers/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: libcpp-has-no-localization
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// REQUIRES: locale.fr_FR.UTF-8
+// REQUIRES: locale.ja_JP.UTF-8
+
+// <chrono>
+// class year;
+
+// template<class charT, class traits>
+//   basic_ostream<charT, traits>&
+//     operator<<(basic_ostream<charT, traits>& os, const year& year);
+
+#include <chrono>
+#include <cassert>
+#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>
+static std::basic_string<CharT> stream_c_locale(std::chrono::year year) {
+  std::basic_stringstream<CharT> sstr;
+  sstr << year;
+  return sstr.str();
+}
+
+template <class CharT>
+static std::basic_string<CharT> stream_fr_FR_locale(std::chrono::year year) {
+  std::basic_stringstream<CharT> sstr;
+  const std::locale locale(LOCALE_fr_FR_UTF_8);
+  sstr.imbue(locale);
+  sstr << year;
+  return sstr.str();
+}
+
+template <class CharT>
+static std::basic_string<CharT> stream_ja_JP_locale(std::chrono::year year) {
+  std::basic_stringstream<CharT> sstr;
+  const std::locale locale(LOCALE_ja_JP_UTF_8);
+  sstr.imbue(locale);
+  sstr << year;
+  return sstr.str();
+}
+
+template <class CharT>
+static void test() {
+  assert(stream_c_locale<CharT>(std::chrono::year{-32'768}) == SV("-32768 is not a valid year"));
+  assert(stream_c_locale<CharT>(std::chrono::year{-32'767}) == SV("-32767"));
+  assert(stream_c_locale<CharT>(std::chrono::year{0}) == SV("0000"));
+  assert(stream_c_locale<CharT>(std::chrono::year{1970}) == SV("1970"));
+  assert(stream_c_locale<CharT>(std::chrono::year{32'767}) == SV("32767"));
+
+  assert(stream_fr_FR_locale<CharT>(std::chrono::year{-32'768}) == SV("-32768 is not a valid year"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::year{-32'767}) == SV("-32767"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::year{0}) == SV("0000"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::year{1970}) == SV("1970"));
+  assert(stream_fr_FR_locale<CharT>(std::chrono::year{32'767}) == SV("32767"));
+
+  assert(stream_ja_JP_locale<CharT>(std::chrono::year{-32'768}) == SV("-32768 is not a valid year"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::year{-32'767}) == SV("-32767"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::year{0}) == SV("0000"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::year{1970}) == SV("1970"));
+  assert(stream_ja_JP_locale<CharT>(std::chrono::year{32'767}) == SV("32767"));
+}
+
+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.year.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: libcpp-has-no-localization
+// UNSUPPORTED: libcpp-has-no-incomplete-format
+
+// REQUIRES: locale.fr_FR.UTF-8
+// REQUIRES: locale.ja_JP.UTF-8
+
+// <chrono>
+
+// template<class charT> struct formatter<chrono::year, 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 "string_literal.h"
+#include "test_macros.h"
+
+template <class CharT>
+static void test_no_chrono_specs() {
+  check.template operator()<"{}">(SV("-32767"), std::chrono::year{-32'767});
+  check.template operator()<"{}">(SV("-1000"), std::chrono::year{-1000});
+  check.template operator()<"{}">(SV("-0100"), std::chrono::year{-100});
+  check.template operator()<"{}">(SV("-0010"), std::chrono::year{-10});
+  check.template operator()<"{}">(SV("-0001"), std::chrono::year{-1});
+  check.template operator()<"{}">(SV("0000"), std::chrono::year{0});
+  check.template operator()<"{}">(SV("0001"), std::chrono::year{1});
+  check.template operator()<"{}">(SV("0010"), std::chrono::year{10});
+  check.template operator()<"{}">(SV("0100"), std::chrono::year{100});
+  check.template operator()<"{}">(SV("1000"), std::chrono::year{1000});
+  check.template operator()<"{}">(SV("32727"), std::chrono::year{32'727});
+
+  // Invalid year
+  check.template operator()<"{}">(SV("-32768 is not a valid year"), std::chrono::year{-32'768});
+  check.template operator()<"{}">(SV("-32768 is not a valid year"), std::chrono::year{32'768});
+}
+
+template <class CharT>
+static void test_valid_values() {
+  constexpr string_literal fmt{
+      "{:"
+      "%%C='%C'%t"
+      "%%EC='%EC'%t"
+      "%%y='%y'%t"
+      "%%Ey='%Ey'%t"
+      "%%Oy='%Oy'%t"
+      "%%Y='%Y'%t"
+      "%%EY='%EY'%t"
+      "%n}"};
+  constexpr string_literal lfmt{
+      "{:L"
+      "%%C='%C'%t"
+      "%%EC='%EC'%t"
+      "%%y='%y'%t"
+      "%%Ey='%Ey'%t"
+      "%%Oy='%Oy'%t"
+      "%%Y='%Y'%t"
+      "%%EY='%EY'%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.template operator()<fmt>(
+      SV("%C='0'\t"
+         "%EC='0'\t"
+         "%y='00'\t"
+         "%Ey='00'\t"
+         "%Oy='00'\t"
+         "%Y='0000'\t"
+         "%EY='0'\t"
+         "\n"),
+      std::chrono::year{0});
+
+  check.template operator()<fmt>(
+      SV("%C='19'\t"
+         "%EC='19'\t"
+         "%y='70'\t"
+         "%Ey='70'\t"
+         "%Oy='70'\t"
+         "%Y='1970'\t"
+         "%EY='1970'\t"
+         "\n"),
+      std::chrono::year{1970});
+
+  check.template operator()<fmt>(
+      SV("%C='20'\t"
+         "%EC='20'\t"
+         "%y='38'\t"
+         "%Ey='38'\t"
+         "%Oy='38'\t"
+         "%Y='2038'\t"
+         "%EY='2038'\t"
+         "\n"),
+      std::chrono::year{2038});
+
+  // Use the global locale (fr_FR)
+  check.template operator()<lfmt>(
+      SV("%C='0'\t"
+         "%EC='0'\t"
+         "%y='00'\t"
+         "%Ey='00'\t"
+         "%Oy='00'\t"
+         "%Y='0000'\t"
+         "%EY='0'\t"
+         "\n"),
+      std::chrono::year{0});
+
+  check.template operator()<lfmt>(
+      SV("%C='19'\t"
+         "%EC='19'\t"
+         "%y='70'\t"
+         "%Ey='70'\t"
+         "%Oy='70'\t"
+         "%Y='1970'\t"
+         "%EY='1970'\t"
+         "\n"),
+      std::chrono::year{1970});
+
+  check.template operator()<lfmt>(
+      SV("%C='20'\t"
+         "%EC='20'\t"
+         "%y='38'\t"
+         "%Ey='38'\t"
+         "%Oy='38'\t"
+         "%Y='2038'\t"
+         "%EY='2038'\t"
+         "\n"),
+      std::chrono::year{2038});
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+  lcheck.template operator()<lfmt>(
+      loc,
+      SV("%C='0'\t"
+         "%EC='紀元前'\t"
+         "%y='00'\t"
+// https://sourceware.org/bugzilla/show_bug.cgi?id=23758
+#if defined(__GLIBC__) && __GLIBC__ <= 2 && __GLIBC_MINOR__ < 29
+         "%Ey='1'\t"
+#else
+         "%Ey='01'\t"
+#endif
+         "%Oy='〇'\t"
+         "%Y='0000'\t"
+// https://sourceware.org/bugzilla/show_bug.cgi?id=23758
+#if defined(__GLIBC__) && __GLIBC__ <= 2 && __GLIBC_MINOR__ < 29
+         "%EY='紀元前1年'\t"
+#else
+         "%EY='紀元前01年'\t"
+#endif
+         "\n"),
+      std::chrono::year{0});
+
+  lcheck.template operator()<lfmt>(
+      loc,
+      SV("%C='19'\t"
+         "%EC='昭和'\t"
+         "%y='70'\t"
+         "%Ey='45'\t"
+         "%Oy='七十'\t"
+         "%Y='1970'\t"
+         "%EY='昭和45年'\t"
+         "\n"),
+      std::chrono::year{1970});
+
+  // Note this test will fail if the Reiwa era ends before 2038.
+  lcheck.template operator()<lfmt>(
+      loc,
+      SV("%C='20'\t"
+         "%EC='令和'\t"
+         "%y='38'\t"
+         "%Ey='20'\t"
+         "%Oy='三十八'\t"
+         "%Y='2038'\t"
+         "%EY='令和20年'\t"
+         "\n"),
+      std::chrono::year{2038});
+
+  std::locale::global(std::locale::classic());
+}
+
+template <class CharT>
+static void test() {
+  test_no_chrono_specs<CharT>();
+  test_valid_values<CharT>();
+  check_invalid_types<CharT>(
+      {SV("C"), SV("y"), SV("Y"), SV("EC"), SV("Ey"), SV("EY"), SV("Oy")}, std::chrono::year{1970});
+
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:A"), std::chrono::year{1970});
+  check_exception("The chrono-specs contains a '{'", SV("{:%%{"), std::chrono::year{1970});
+  check_exception("End of input while parsing the modifier chrono conversion-spec", SV("{:%"), std::chrono::year{1970});
+  check_exception("End of input while parsing the modifier E", SV("{:%E"), std::chrono::year{1970});
+  check_exception("End of input while parsing the modifier O", SV("{:%O"), std::chrono::year{1970});
+
+  // Precision not allowed
+  check_exception("Expected '%' or '}' in the chrono format-string", SV("{:.3}"), std::chrono::year{1970});
+}
+
+int main(int, char**) {
+  test<char>();
+
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+  test<wchar_t>();
+#endif
+
+  return 0;
+}