diff --git a/libcxx/include/__format/unicode.h b/libcxx/include/__format/unicode.h
--- a/libcxx/include/__format/unicode.h
+++ b/libcxx/include/__format/unicode.h
@@ -87,17 +87,15 @@
 }
 
 // https://www.unicode.org/glossary/#surrogate_code_point
-[[nodiscard]] _LIBCPP_HIDE_FROM_ABI inline constexpr bool __is_surrogate(char32_t __value) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool __is_surrogate(char32_t __value) {
   return __value >= 0xd800 && __value <= 0xdfff;
 }
 
 // https://www.unicode.org/glossary/#code_point
-[[nodiscard]] _LIBCPP_HIDE_FROM_ABI inline constexpr bool __is_code_point(char32_t __value) {
-  return __value <= 0x10ffff;
-}
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool __is_code_point(char32_t __value) { return __value <= 0x10ffff; }
 
 // https://www.unicode.org/glossary/#unicode_scalar_value
-[[nodiscard]] _LIBCPP_HIDE_FROM_ABI inline constexpr bool __is_scalar_value(char32_t __value) {
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool __is_scalar_value(char32_t __value) {
   return __unicode::__is_code_point(__value) && !__unicode::__is_surrogate(__value);
 }
 
diff --git a/libcxx/test/std/time/time.syn/formatter.hh_mm_ss.pass.cpp b/libcxx/test/std/time/time.syn/formatter.hh_mm_ss.pass.cpp
--- a/libcxx/test/std/time/time.syn/formatter.hh_mm_ss.pass.cpp
+++ b/libcxx/test/std/time/time.syn/formatter.hh_mm_ss.pass.cpp
@@ -9,9 +9,6 @@
 // UNSUPPORTED: no-localization
 // UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
 
-// TODO FMT Investigate Windows issues.
-// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
-
 // XFAIL: LIBCXX-FREEBSD-FIXME
 
 // XFAIL: availability-fp_to_chars-missing
@@ -293,7 +290,7 @@
         std::chrono::hh_mm_ss(std::chrono::duration<double>(3661.123456)));
 
   // Use supplied locale (ja_JP). This locale has a different alternate.
-#if defined(__APPLE__) || defined(_AIX)
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("%H='00'\t"
            "%OH='00'\t"
@@ -314,6 +311,10 @@
            "%r='12:00:00 AM'\t"
            "%X='00時00分00秒'\t"
            "%EX='00時00分00秒'\t"
+#  elif defined(_WIN32)
+           "%r='0:00:00'\t"
+           "%X='0:00:00'\t"
+           "%EX='0:00:00'\t"
 #  else
            "%r='午前12:00:00'\t"
            "%X='00:00:00'\t"
@@ -343,6 +344,10 @@
            "%r='11:31:30 PM'\t"
            "%X='23時31分30秒'\t"
            "%EX='23時31分30秒'\t"
+#  elif defined(_WIN32)
+           "%r='23:31:30'\t"
+           "%X='23:31:30'\t"
+           "%EX='23:31:30'\t"
 #  else
            "%r='午後11:31:30'\t"
            "%X='23:31:30'\t"
@@ -372,6 +377,10 @@
            "%r='03:02:01 AM'\t"
            "%X='03時02分01秒'\t"
            "%EX='03時02分01秒'\t"
+#  elif defined(_WIN32)
+           "%r='3:02:01'\t"
+           "%X='3:02:01'\t"
+           "%EX='3:02:01'\t"
 #  else
            "%r='午前03:02:01'\t"
            "%X='03:02:01'\t"
@@ -401,6 +410,10 @@
            "%r='01:01:01 AM'\t"
            "%X='01時01分01秒'\t"
            "%EX='01時01分01秒'\t"
+#  elif defined(_WIN32)
+           "%r='1:01:01'\t"
+           "%X='1:01:01'\t"
+           "%EX='1:01:01'\t"
 #  else
            "%r='午前01:01:01'\t"
            "%X='01:01:01'\t"
@@ -409,7 +422,7 @@
            "\n"),
         lfmt,
         std::chrono::hh_mm_ss(std::chrono::duration<double>(3661.123456)));
-#else  // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("%H='00'\t"
            "%OH='〇'\t"
@@ -486,7 +499,7 @@
            "\n"),
         lfmt,
         std::chrono::hh_mm_ss(std::chrono::duration<double>(3661.123456)));
-#endif // defined(__APPLE__) || defined(_AIX)
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
 
   std::locale::global(std::locale::classic());
 }
diff --git a/libcxx/test/std/time/time.syn/formatter.day.pass.cpp b/libcxx/test/std/time/time.syn/formatter.day.pass.cpp
--- a/libcxx/test/std/time/time.syn/formatter.day.pass.cpp
+++ b/libcxx/test/std/time/time.syn/formatter.day.pass.cpp
@@ -11,12 +11,12 @@
 // UNSUPPORTED: no-localization
 // UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
 
-// TODO FMT Investigate Windows issues.
-// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
-
 // TODO FMT This test should not require std::to_chars(floating-point)
 // XFAIL: availability-fp_to_chars-missing
 
+// TODO FMT Investigate Windows issues.
+// XFAIL: msvc
+
 // REQUIRES: locale.fr_FR.UTF-8
 // REQUIRES: locale.ja_JP.UTF-8
 
@@ -64,41 +64,59 @@
   std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
 
   // Non localized output using C-locale
+#if defined(_WIN32)
+  check(SV("%d=''\t%Od=''\t%e=''\t%Oe=''\n"), fmt, 0d);
+#else
   check(SV("%d='00'\t%Od='00'\t%e=' 0'\t%Oe=' 0'\n"), fmt, 0d);
+#endif
   check(SV("%d='01'\t%Od='01'\t%e=' 1'\t%Oe=' 1'\n"), fmt, 1d);
   check(SV("%d='31'\t%Od='31'\t%e='31'\t%Oe='31'\n"), fmt, 31d);
-#if defined(_AIX)
+#if defined(_WIN32)
+  check(SV("%d=''\t%Od=''\t%e=''\t%Oe=''\n"), fmt, 0d);
+#elif defined(_AIX)
   check(SV("%d='55'\t%Od='55'\t%e='55'\t%Oe='55'\n"), fmt, 255d);
 #else
   check(SV("%d='255'\t%Od='255'\t%e='255'\t%Oe='255'\n"), fmt, 255d);
 #endif
 
   // Use the global locale (fr_FR)
+#if defined(_WIN32)
+  check(SV("%d=''\t%Od=''\t%e=''\t%Oe=''\n"), lfmt, 0d);
+#else
   check(SV("%d='00'\t%Od='00'\t%e=' 0'\t%Oe=' 0'\n"), lfmt, 0d);
+#endif
   check(SV("%d='01'\t%Od='01'\t%e=' 1'\t%Oe=' 1'\n"), lfmt, 1d);
   check(SV("%d='31'\t%Od='31'\t%e='31'\t%Oe='31'\n"), lfmt, 31d);
-#if defined(_AIX)
+#if defined(_WIN32)
+  check(SV("%d=''\t%Od=''\t%e=''\t%Oe=''\n"), lfmt, 255d);
+#elif defined(_AIX)
   check(SV("%d='55'\t%Od='55'\t%e='55'\t%Oe='55'\n"), lfmt, 255d);
 #else
   check(SV("%d='255'\t%Od='255'\t%e='255'\t%Oe='255'\n"), lfmt, 255d);
 #endif
 
   // Use supplied locale (ja_JP). This locale has a different alternate on some platforms.
-#if defined(__APPLE__) || defined(_AIX)
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
+#  if defined(_WIN32)
+  check(loc, SV("%d=''\t%Od=''\t%e=''\t%Oe=''\n"), lfmt, 0d);
+#  else
   check(loc, SV("%d='00'\t%Od='00'\t%e=' 0'\t%Oe=' 0'\n"), lfmt, 0d);
+#  endif
   check(loc, SV("%d='01'\t%Od='01'\t%e=' 1'\t%Oe=' 1'\n"), lfmt, 1d);
   check(loc, SV("%d='31'\t%Od='31'\t%e='31'\t%Oe='31'\n"), lfmt, 31d);
-#  if defined(_AIX)
+#  if defined(_WIN32)
+  check(SV("%d=''\t%Od=''\t%e=''\t%Oe=''\n"), fmt, 255d);
+#  elif defined(_AIX)
   check(SV("%d='55'\t%Od='55'\t%e='55'\t%Oe='55'\n"), fmt, 255d);
 #  else
   check(SV("%d='255'\t%Od='255'\t%e='255'\t%Oe='255'\n"), fmt, 255d);
 #  endif
-#else  // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc, SV("%d='00'\t%Od='〇'\t%e=' 0'\t%Oe='〇'\n"), lfmt, 0d);
   check(loc, SV("%d='01'\t%Od='一'\t%e=' 1'\t%Oe='一'\n"), lfmt, 1d);
   check(loc, SV("%d='31'\t%Od='三十一'\t%e='31'\t%Oe='三十一'\n"), lfmt, 31d);
   check(loc, SV("%d='255'\t%Od='255'\t%e='255'\t%Oe='255'\n"), lfmt, 255d);
-#endif // defined(__APPLE__) || defined(_AIX)
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
 
   std::locale::global(std::locale::classic());
 }
diff --git a/libcxx/test/std/time/time.syn/formatter.duration.pass.cpp b/libcxx/test/std/time/time.syn/formatter.duration.pass.cpp
--- a/libcxx/test/std/time/time.syn/formatter.duration.pass.cpp
+++ b/libcxx/test/std/time/time.syn/formatter.duration.pass.cpp
@@ -11,9 +11,6 @@
 // UNSUPPORTED: no-localization
 // UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
 
-// TODO FMT Investigate Windows issues.
-// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
-
 // XFAIL: availability-fp_to_chars-missing
 
 // REQUIRES: locale.fr_FR.UTF-8
@@ -247,7 +244,7 @@
            "%R='00:00'\t"
            "%T='00:00:00'\t"
 #ifdef _WIN32
-           "%r='12:00:00'\t"
+           "%r='00:00:00'\t"
 #elif defined(_AIX)
            "%r='12:00:00 AM'\t"
 #elif defined(__APPLE__)
@@ -313,7 +310,7 @@
            "%R='12:00'\t"
            "%T='12:00:00'\t"
 #ifdef _WIN32
-           "%r='00:00:00'\t"
+           "%r='12:00:00'\t"
 #elif defined(_AIX)
            "%r='12:00:00 PM'\t"
 #elif defined(__APPLE__)
@@ -349,6 +346,8 @@
            "%r='11:59:59 PM'\t"
 #elif defined(__APPLE__)
            "%r=''\t"
+#elif defined(_WIN32)
+           "%r='23:59:59'\t"
 #else
            "%r='11:59:59 '\t"
 #endif
@@ -377,11 +376,13 @@
            "%R='00:00'\t"
            "%T='00:00:00'\t"
 #ifdef _WIN32
-           "%r='12:00:00'\t"
+           "%r='00:00:00'\t"
 #elif defined(_AIX)
            "%r='12:00:00 AM'\t"
 #elif defined(__APPLE__)
            "%r=''\t"
+#elif defined(_WIN32)
+           "%r='12:00:00'\t"
 #else
            "%r='12:00:00 '\t"
 #endif
@@ -395,7 +396,7 @@
         std::chrono::duration<int, std::ratio<86400>>(7));
 
   // Use supplied locale (ja_JP). This locale has a different alternate.
-#if defined(__APPLE__) || defined(_AIX)
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("%H='00'\t"
            "%OH='00'\t"
@@ -416,6 +417,10 @@
            "%r='12:00:00 AM'\t"
            "%X='00時00分00秒'\t"
            "%EX='00時00分00秒'\t"
+#  elif defined(_WIN32)
+           "%r='0:00:00'\t"
+           "%X='0:00:00'\t"
+           "%EX='0:00:00'\t"
 #  else
            "%r='午前12:00:00'\t"
            "%X='00:00:00'\t"
@@ -448,6 +453,10 @@
            "%r='11:59:59 AM'\t"
            "%X='11時59分59秒'\t"
            "%EX='11時59分59秒'\t"
+#  elif defined(_WIN32)
+           "%r='11:59:59'\t"
+           "%X='11:59:59'\t"
+           "%EX='11:59:59'\t"
 #  else
            "%r='午前11:59:59'\t"
            "%X='11:59:59'\t"
@@ -481,7 +490,11 @@
            "%X='12時00分00秒'\t"
            "%EX='12時00分00秒'\t"
 #  else
+#    ifdef _WIN32
+           "%r='12:00:00'\t"
+#    else
            "%r='午後12:00:00'\t"
+#    endif
            "%X='12:00:00'\t"
            "%EX='12:00:00'\t"
 #  endif
@@ -513,7 +526,11 @@
            "%X='23時59分59秒'\t"
            "%EX='23時59分59秒'\t"
 #  else
+#    ifdef _WIN32
+           "%r='23:59:59'\t"
+#    else
            "%r='午後11:59:59'\t"
+#    endif
            "%X='23:59:59'\t"
            "%EX='23:59:59'\t"
 #  endif
@@ -544,6 +561,10 @@
            "%r='12:00:00 AM'\t"
            "%X='00時00分00秒'\t"
            "%EX='00時00分00秒'\t"
+#  elif defined(_WIN32)
+           "%r='0:00:00'\t"
+           "%X='0:00:00'\t"
+           "%EX='0:00:00'\t"
 #  else
            "%r='午前12:00:00'\t"
            "%X='00:00:00'\t"
@@ -555,7 +576,7 @@
            "\n"),
         lfmt,
         std::chrono::duration<int, std::ratio<86400>>(7));
-#else // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("%H='00'\t"
            "%OH='〇'\t"
@@ -665,8 +686,8 @@
            "\n"),
         lfmt,
         std::chrono::duration<int, std::ratio<86400>>(7));
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
 
-#endif // defined(__APPLE__) || defined(_AIX)
   std::locale::global(std::locale::classic());
 }
 
@@ -767,6 +788,8 @@
            "%r='11:59:59 PM'\t"
 #elif defined(__APPLE__)
            "%r=''\t"
+#elif defined(_WIN32)
+           "%r='23:59:59'\t"
 #else
            "%r='11:59:59 '\t"
 #endif
@@ -780,7 +803,7 @@
         -(23h + 59min + 59s));
 
   // Use supplied locale (ja_JP). This locale has a different alternate.
-#if defined(__APPLE__) || defined(_AIX)
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("-%H='23'\t"
            "%OH='23'\t"
@@ -801,6 +824,10 @@
            "%r='11:59:59 PM'\t"
            "%X='23時59分59秒'\t"
            "%EX='23時59分59秒'\t"
+#  elif defined(_WIN32)
+           "%r='23:59:59'\t"
+           "%X='23:59:59'\t"
+           "%EX='23:59:59'\t"
 #  else
            "%r='午後11:59:59'\t"
            "%X='23:59:59'\t"
@@ -812,7 +839,7 @@
            "\n"),
         lfmt,
         -(23h + 59min + 59s));
-#else  // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("-%H='23'\t"
            "%OH='二十三'\t"
@@ -834,7 +861,7 @@
            "\n"),
         lfmt,
         -(23h + 59min + 59s));
-#endif // defined(__APPLE__) || defined(_AIX)
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   std::locale::global(std::locale::classic());
 }
 
@@ -987,7 +1014,7 @@
   check(loc, SV("00.011"), SV("{:L%S}"), std::chrono::duration<double, std::milli>(11.123456789));
   check(loc, SV("01"), SV("{:L%S}"), std::chrono::duration<long double>(61.123456789));
 
-#if defined(__APPLE__) || defined(_AIX)
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(SV("00.000000001"), SV("{:%OS}"), 1ns);
   check(SV("00.000000501"), SV("{:%OS}"), 501ns);
   check(SV("00.000001000"), SV("{:%OS}"), 1000ns);
@@ -1003,7 +1030,7 @@
   check(SV("00.001"), SV("{:%OS}"), std::chrono::duration<float, std::milli>(1.123456789));
   check(SV("00.011"), SV("{:%OS}"), std::chrono::duration<double, std::milli>(11.123456789));
   check(SV("01"), SV("{:%OS}"), std::chrono::duration<long double>(61.123456789));
-#else  // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc, SV("〇.000000001"), SV("{:L%OS}"), 1ns);
   check(loc, SV("〇.000000501"), SV("{:L%OS}"), 501ns);
   check(loc, SV("〇.000001000"), SV("{:L%OS}"), 1000ns);
@@ -1019,7 +1046,7 @@
   check(loc, SV("〇.001"), SV("{:L%OS}"), std::chrono::duration<float, std::milli>(1.123456789));
   check(loc, SV("〇.011"), SV("{:L%OS}"), std::chrono::duration<double, std::milli>(11.123456789));
   check(loc, SV("一"), SV("{:L%OS}"), std::chrono::duration<long double>(61.123456789));
-#endif // defined(__APPLE__) || defined(_AIX)
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
 
   check(loc, SV("01:05:06.000000001"), SV("{:L%T}"), 1h + 5min + 6s + 1ns);
   check(loc, SV("01:05:06.000000501"), SV("{:L%T}"), 1h + 5min + 6s + 501ns);
diff --git a/libcxx/test/std/time/time.syn/formatter.file_time.pass.cpp b/libcxx/test/std/time/time.syn/formatter.file_time.pass.cpp
--- a/libcxx/test/std/time/time.syn/formatter.file_time.pass.cpp
+++ b/libcxx/test/std/time/time.syn/formatter.file_time.pass.cpp
@@ -14,9 +14,6 @@
 // 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
-
 // REQUIRES: locale.fr_FR.UTF-8
 // REQUIRES: locale.ja_JP.UTF-8
 
@@ -245,7 +242,7 @@
         lfmt,
         file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
 
-#endif                                 // _WIN32
+#endif // _WIN32
 
   std::locale::global(std::locale::classic());
 }
@@ -336,7 +333,7 @@
   check(SV("%j='138'\n"), lfmt, file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
 
   // Use supplied locale (ja_JP). This locale has a different alternate.
-  check(loc, SV("%j='001'\n"), lfmt, file_seconds(0s));             // 00:00:00 UTC Thursday, 1 January 1970
+  check(loc, SV("%j='001'\n"), lfmt, file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
 
   check(loc, SV("%j='138'\n"), lfmt, file_seconds(2'000'000'000s)); // 03:33:20 UTC on Wednesday, 18 May 2033
 
@@ -641,6 +638,8 @@
            "%r='11:31:30 PM'\t"
 #elif defined(__APPLE__)
            "%r=''\t"
+#elif defined(_WIN32)
+           "%r='23:31:30 '\t"
 #else
            "%r='11:31:30 '\t"
 #endif
@@ -652,7 +651,7 @@
             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)
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("%H='00'\t"
            "%OH='00'\t"
@@ -673,6 +672,10 @@
            "%r='12:00:00 AM'\t"
            "%X='00時00分00秒'\t"
            "%EX='00時00分00秒'\t"
+#  elif defined(_WIN32)
+           "%r='0:00:00'\t"
+           "%X='0:00:00'\t"
+           "%EX='0:00:00'\t"
 #  else
            "%r='午前12:00:00'\t"
            "%X='00:00:00'\t"
@@ -702,6 +705,10 @@
            "%r='11:31:30 PM'\t"
            "%X='23時31分30秒'\t"
            "%EX='23時31分30秒'\t"
+#  elif defined(_WIN32)
+           "%r='23:31:30'\t"
+           "%X='23:31:30'\t"
+           "%EX='23:31:30'\t"
 #  else
            "%r='午後11:31:30'\t"
            "%X='23:31:30'\t"
@@ -710,7 +717,7 @@
            "\n"),
         lfmt,
         std::chrono::hh_mm_ss(23h + 31min + 30s + 123ms));
-#else  // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("%H='00'\t"
            "%OH='〇'\t"
@@ -749,7 +756,7 @@
         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)
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
 
   std::locale::global(std::locale::classic());
 }
@@ -782,6 +789,8 @@
       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"),
+#elif defined(_WIN32)
+      SV("%c='01/01/1970 00:00:00'\t%Ec='01/01/1970 00:00:00'\n"),
 #else
       SV("%c='jeu. 01 janv. 1970 00:00:00'\t%Ec='jeu. 01 janv. 1970 00:00:00'\n"),
 #endif
@@ -796,6 +805,8 @@
       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"),
+#elif defined(_WIN32)
+      SV("%c='13/02/2009 23:31:30'\t%Ec='13/02/2009 23:31:30'\n"),
 #else
       SV("%c='ven. 13 févr. 2009 23:31:30'\t%Ec='ven. 13 févr. 2009 23:31:30'\n"),
 #endif
@@ -821,6 +832,15 @@
         SV("%c='2009年02月13日 23:31:30 UTC'\t%Ec='2009年02月13日 23:31:30 UTC'\n"),
         lfmt,
         file_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#elif defined(_WIN32)                  // __APPLE__
+  check(loc,
+        SV("%c='1970/01/01 0:00:00'\t%Ec='1970/01/01 0:00:00'\n"),
+        lfmt,
+        std::chrono::sys_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+  check(loc,
+        SV("%c='2009/02/13 23:31:30'\t%Ec='2009/02/13 23:31:30'\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"),
@@ -866,6 +886,22 @@
         SV("%z='UTC'\t%Ez='UTC'\t%Oz='UTC'\t%Z='UTC'\n"),
         lfmt,
         file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+#  elif defined(_WIN32)    // defined(_AIX)
+  // Non localized output using C-locale
+  check(SV("%z='-0000'\t%Ez='-0000'\t%Oz='-0000'\t%Z='UTC'\n"),
+        fmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  // Use the global locale (fr_FR)
+  check(SV("%z='-0000'\t%Ez='-0000'\t%Oz='-0000'\t%Z='UTC'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.a
+  check(loc,
+        SV("%z='-0000'\t%Ez='-0000'\t%Oz='-0000'\t%Z='UTC'\n"),
+        lfmt,
+        file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
 #  else                    // defined(_AIX)
   // Non localized output using C-locale
   check(SV("%z='+0000'\t%Ez='+0000'\t%Oz='+0000'\t%Z='UTC'\n"),
@@ -884,7 +920,7 @@
         file_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
 #  endif                   // defined(_AIX)
   std::locale::global(std::locale::classic());
-#endif                     // !defined(__APPLE__)
+#endif // !defined(__APPLE__)
 }
 
 template <class CharT>
diff --git a/libcxx/test/std/time/time.syn/formatter.local_time.pass.cpp b/libcxx/test/std/time/time.syn/formatter.local_time.pass.cpp
--- a/libcxx/test/std/time/time.syn/formatter.local_time.pass.cpp
+++ b/libcxx/test/std/time/time.syn/formatter.local_time.pass.cpp
@@ -14,9 +14,6 @@
 // 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
-
 // REQUIRES: locale.fr_FR.UTF-8
 // REQUIRES: locale.ja_JP.UTF-8
 
@@ -240,7 +237,7 @@
         lfmt,
         std::chrono::local_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
 
-#endif                                               // _WIN32
+#endif // _WIN32
 
   std::locale::global(std::locale::classic());
 }
@@ -654,7 +651,7 @@
             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)
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("%H='00'\t"
            "%OH='00'\t"
@@ -675,6 +672,10 @@
            "%r='12:00:00 AM'\t"
            "%X='00時00分00秒'\t"
            "%EX='00時00分00秒'\t"
+#  elif defined(_WIN32)
+           "%r='0:00:00'\t"
+           "%X='0:00:00'\t"
+           "%EX='0:00:00'\t"
 #  else
            "%r='午前12:00:00'\t"
            "%X='00:00:00'\t"
@@ -704,6 +705,10 @@
            "%r='11:31:30 PM'\t"
            "%X='23時31分30秒'\t"
            "%EX='23時31分30秒'\t"
+#  elif defined(_WIN32)
+           "%r='23:31:30'\t"
+           "%X='23:31:30'\t"
+           "%EX='23:31:30'\t"
 #  else
            "%r='午後11:31:30'\t"
            "%X='23:31:30'\t"
@@ -712,7 +717,7 @@
            "\n"),
         lfmt,
         std::chrono::hh_mm_ss(23h + 31min + 30s + 123ms));
-#else  // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("%H='00'\t"
            "%OH='〇'\t"
@@ -751,7 +756,7 @@
         lfmt,
         std::chrono::local_time<std::chrono::milliseconds>(
             1'234'567'890'123ms)); // 23:31:30 UTC on Friday, 13 February 2009
-#endif // defined(__APPLE__) || defined(_AIX)
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
 
   std::locale::global(std::locale::classic());
 }
@@ -784,6 +789,8 @@
       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"),
+#elif defined(_WIN32)
+      SV("%c='01/01/1970 00:00:00'\t%Ec='01/01/1970 00:00:00'\n"),
 #else
       SV("%c='jeu. 01 janv. 1970 00:00:00'\t%Ec='jeu. 01 janv. 1970 00:00:00'\n"),
 #endif
@@ -798,6 +805,8 @@
       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"),
+#elif defined(_WIN32)
+      SV("%c='13/02/2009 23:31:30'\t%Ec='13/02/2009 23:31:30'\n"),
 #else
       SV("%c='ven. 13 févr. 2009 23:31:30'\t%Ec='ven. 13 févr. 2009 23:31:30'\n"),
 #endif
@@ -823,6 +832,15 @@
         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::local_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
+#elif defined(_WIN32)                                // __APPLE__
+  check(loc,
+        SV("%c='1970/01/01 0:00:00'\t%Ec='1970/01/01 0:00:00'\n"),
+        lfmt,
+        std::chrono::sys_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+  check(loc,
+        SV("%c='2009/02/13 23:31:30'\t%Ec='2009/02/13 23:31:30'\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"),
diff --git a/libcxx/test/std/time/time.syn/formatter.month_day.pass.cpp b/libcxx/test/std/time/time.syn/formatter.month_day.pass.cpp
--- a/libcxx/test/std/time/time.syn/formatter.month_day.pass.cpp
+++ b/libcxx/test/std/time/time.syn/formatter.month_day.pass.cpp
@@ -11,12 +11,12 @@
 // UNSUPPORTED: no-localization
 // UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
 
-// TODO FMT Investigate Windows issues.
-// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
-
 // TODO FMT This test should not require std::to_chars(floating-point)
 // XFAIL: availability-fp_to_chars-missing
 
+// TODO FMT Investigate Windows issues.
+// XFAIL: msvc
+
 // REQUIRES: locale.fr_FR.UTF-8
 // REQUIRES: locale.ja_JP.UTF-8
 
@@ -112,9 +112,15 @@
   std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
 
   // Non localized output using C-locale
+#ifdef _WIN32
+  check(SV("%b='Jan'\t%B='January'\t%h='Jan'\t%m='01'\t%Om='01'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        fmt,
+        std::chrono::month_day{std::chrono::January, std::chrono::day{0}});
+#else
   check(SV("%b='Jan'\t%B='January'\t%h='Jan'\t%m='01'\t%Om='01'\t%d='00'\t%e=' 0'\t%Od='00'\t%Oe=' 0'\n"),
         fmt,
         std::chrono::month_day{std::chrono::January, std::chrono::day{0}});
+#endif
   check(SV("%b='Feb'\t%B='February'\t%h='Feb'\t%m='02'\t%Om='02'\t%d='01'\t%e=' 1'\t%Od='01'\t%Oe=' 1'\n"),
         fmt,
         std::chrono::month_day{std::chrono::February, std::chrono::day{1}});
@@ -136,27 +142,42 @@
   check(SV("%b='Aug'\t%B='August'\t%h='Aug'\t%m='08'\t%Om='08'\t%d='31'\t%e='31'\t%Od='31'\t%Oe='31'\n"),
         fmt,
         std::chrono::month_day{std::chrono::August, std::chrono::day{31}});
+#ifdef _WIN32
+  check(SV("%b='Sep'\t%B='September'\t%h='Sep'\t%m='09'\t%Om='09'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        fmt,
+        std::chrono::month_day{std::chrono::September, std::chrono::day{32}});
+  check(SV("%b='Oct'\t%B='October'\t%h='Oct'\t%m='10'\t%Om='10'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        fmt,
+        std::chrono::month_day{std::chrono::October, std::chrono::day{99}});
+  check(SV("%b='Nov'\t%B='November'\t%h='Nov'\t%m='11'\t%Om='11'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        fmt,
+        std::chrono::month_day{std::chrono::November, std::chrono::day{100}});
+  check(SV("%b='Dec'\t%B='December'\t%h='Dec'\t%m='12'\t%Om='12'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        fmt,
+        std::chrono::month_day{std::chrono::December, std::chrono::day{255}});
+#else // _WIN32
   check(SV("%b='Sep'\t%B='September'\t%h='Sep'\t%m='09'\t%Om='09'\t%d='32'\t%e='32'\t%Od='32'\t%Oe='32'\n"),
         fmt,
         std::chrono::month_day{std::chrono::September, std::chrono::day{32}});
   check(SV("%b='Oct'\t%B='October'\t%h='Oct'\t%m='10'\t%Om='10'\t%d='99'\t%e='99'\t%Od='99'\t%Oe='99'\n"),
         fmt,
         std::chrono::month_day{std::chrono::October, std::chrono::day{99}});
-#if defined(_AIX)
+#  if defined(_AIX)
   check(SV("%b='Nov'\t%B='November'\t%h='Nov'\t%m='11'\t%Om='11'\t%d='00'\t%e=' 0'\t%Od='00'\t%Oe=' 0'\n"),
         fmt,
         std::chrono::month_day{std::chrono::November, std::chrono::day{100}});
   check(SV("%b='Dec'\t%B='December'\t%h='Dec'\t%m='12'\t%Om='12'\t%d='55'\t%e='55'\t%Od='55'\t%Oe='55'\n"),
         fmt,
         std::chrono::month_day{std::chrono::December, std::chrono::day{255}});
-#else  //  defined(_AIX)
+#  else  //  defined(_AIX)
   check(SV("%b='Nov'\t%B='November'\t%h='Nov'\t%m='11'\t%Om='11'\t%d='100'\t%e='100'\t%Od='100'\t%Oe='100'\n"),
         fmt,
         std::chrono::month_day{std::chrono::November, std::chrono::day{100}});
   check(SV("%b='Dec'\t%B='December'\t%h='Dec'\t%m='12'\t%Om='12'\t%d='255'\t%e='255'\t%Od='255'\t%Oe='255'\n"),
         fmt,
         std::chrono::month_day{std::chrono::December, std::chrono::day{255}});
-#endif //  defined(_AIX)
+#  endif //  defined(_AIX)
+#endif   // _WIN32
 
   // Use the global locale (fr_FR)
 #if defined(__APPLE__)
@@ -196,10 +217,16 @@
   check(SV("%b='déc'\t%B='décembre'\t%h='déc'\t%m='12'\t%Om='12'\t%d='255'\t%e='255'\t%Od='255'\t%Oe='255'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::December, std::chrono::day{255}});
-#else    // defined(__APPLE__)
+#else // defined(__APPLE__)
+#  if defined(_WIN32)
+  check(SV("%b='janv.'\t%B='janvier'\t%h='janv.'\t%m='01'\t%Om='01'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        lfmt,
+        std::chrono::month_day{std::chrono::January, std::chrono::day{0}});
+#  else
   check(SV("%b='janv.'\t%B='janvier'\t%h='janv.'\t%m='01'\t%Om='01'\t%d='00'\t%e=' 0'\t%Od='00'\t%Oe=' 0'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::January, std::chrono::day{0}});
+#  endif
   check(SV("%b='févr.'\t%B='février'\t%h='févr.'\t%m='02'\t%Om='02'\t%d='01'\t%e=' 1'\t%Od='01'\t%Oe=' 1'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::February, std::chrono::day{1}});
@@ -226,77 +253,92 @@
   check(SV("%b='août'\t%B='août'\t%h='août'\t%m='08'\t%Om='08'\t%d='31'\t%e='31'\t%Od='31'\t%Oe='31'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::August, std::chrono::day{31}});
+#  if defined(_WIN32)
+  check(SV("%b='sept.'\t%B='septembre'\t%h='sept.'\t%m='09'\t%Om='09'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        lfmt,
+        std::chrono::month_day{std::chrono::September, std::chrono::day{32}});
+  check(SV("%b='oct.'\t%B='octobre'\t%h='oct.'\t%m='10'\t%Om='10'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        lfmt,
+        std::chrono::month_day{std::chrono::October, std::chrono::day{99}});
+  check(SV("%b='nov.'\t%B='novembre'\t%h='nov.'\t%m='11'\t%Om='11'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        lfmt,
+        std::chrono::month_day{std::chrono::November, std::chrono::day{100}});
+  check(SV("%b='déc.'\t%B='décembre'\t%h='déc.'\t%m='12'\t%Om='12'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        lfmt,
+        std::chrono::month_day{std::chrono::December, std::chrono::day{255}});
+#  else // defined(_WIN32)
   check(SV("%b='sept.'\t%B='septembre'\t%h='sept.'\t%m='09'\t%Om='09'\t%d='32'\t%e='32'\t%Od='32'\t%Oe='32'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::September, std::chrono::day{32}});
   check(SV("%b='oct.'\t%B='octobre'\t%h='oct.'\t%m='10'\t%Om='10'\t%d='99'\t%e='99'\t%Od='99'\t%Oe='99'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::October, std::chrono::day{99}});
-#  if defined(_AIX)
+#    if defined(_AIX)
   check(SV("%b='nov.'\t%B='novembre'\t%h='nov.'\t%m='11'\t%Om='11'\t%d='00'\t%e=' 0'\t%Od='00'\t%Oe=' 0'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::November, std::chrono::day{100}});
   check(SV("%b='déc.'\t%B='décembre'\t%h='déc.'\t%m='12'\t%Om='12'\t%d='55'\t%e='55'\t%Od='55'\t%Oe='55'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::December, std::chrono::day{255}});
-#  else  //   defined(_AIX)
+#    else  //   defined(_AIX)
   check(SV("%b='nov.'\t%B='novembre'\t%h='nov.'\t%m='11'\t%Om='11'\t%d='100'\t%e='100'\t%Od='100'\t%Oe='100'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::November, std::chrono::day{100}});
   check(SV("%b='déc.'\t%B='décembre'\t%h='déc.'\t%m='12'\t%Om='12'\t%d='255'\t%e='255'\t%Od='255'\t%Oe='255'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::December, std::chrono::day{255}});
-#  endif //   defined(_AIX)
-#endif   // defined(__APPLE__)
+#    endif //   defined(_AIX)
+#  endif   //   defined(_WIN32)
+#endif     // defined(__APPLE__)
 
   // Use supplied locale (ja_JP)
 #if defined(_WIN32)
   check(loc,
-        SV("%b='1'\t%B='1月'\t%h='1'\t%m='01'\t%Om='01'\t%d='00'\t%e=' 0'\t%Od='〇'\t%Oe='〇'\n"),
+        SV("%b='1'\t%B='1月'\t%h='1'\t%m='01'\t%Om='01'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
         lfmt,
         std::chrono::month_day{std::chrono::January, std::chrono::day{0}});
   check(loc,
-        SV("%b='2'\t%B='2月'\t%h='2'\t%m='02'\t%Om='02'\t%d='01'\t%e=' 1'\t%Od='一'\t%Oe='一'\n"),
+        SV("%b='2'\t%B='2月'\t%h='2'\t%m='02'\t%Om='02'\t%d='01'\t%e=' 1'\t%Od='01'\t%Oe=' 1'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::February, std::chrono::day{1}});
   check(loc,
-        SV("%b='3'\t%B='3月'\t%h='3'\t%m='03'\t%Om='03'\t%d='09'\t%e=' 9'\t%Od='九'\t%Oe='九'\n"),
+        SV("%b='3'\t%B='3月'\t%h='3'\t%m='03'\t%Om='03'\t%d='09'\t%e=' 9'\t%Od='09'\t%Oe=' 9'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::March, std::chrono::day{9}});
   check(loc,
-        SV("%b='4'\t%B='4月'\t%h='4'\t%m='04'\t%Om='04'\t%d='10'\t%e='10'\t%Od='十'\t%Oe='十'\n"),
+        SV("%b='4'\t%B='4月'\t%h='4'\t%m='04'\t%Om='04'\t%d='10'\t%e='10'\t%Od='10'\t%Oe='10'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::April, std::chrono::day{10}});
   check(loc,
-        SV("%b='5'\t%B='5月'\t%h='5'\t%m='05'\t%Om='05'\t%d='28'\t%e='28'\t%Od='二十八'\t%Oe='二十八'\n"),
+        SV("%b='5'\t%B='5月'\t%h='5'\t%m='05'\t%Om='05'\t%d='28'\t%e='28'\t%Od='28'\t%Oe='28'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::May, std::chrono::day{28}});
   check(loc,
-        SV("%b='6'\t%B='6月'\t%h='6'\t%m='06'\t%Om='06'\t%d='29'\t%e='29'\t%Od='二十九'\t%Oe='二十九'\n"),
+        SV("%b='6'\t%B='6月'\t%h='6'\t%m='06'\t%Om='06'\t%d='29'\t%e='29'\t%Od='29'\t%Oe='29'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::June, std::chrono::day{29}});
   check(loc,
-        SV("%b='7'\t%B='7月'\t%h='7'\t%m='07'\t%Om='07'\t%d='30'\t%e='30'\t%Od='三十'\t%Oe='三十'\n"),
+        SV("%b='7'\t%B='7月'\t%h='7'\t%m='07'\t%Om='07'\t%d='30'\t%e='30'\t%Od='30'\t%Oe='30'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::July, std::chrono::day{30}});
   check(loc,
-        SV("%b='8'\t%B='8月'\t%h='8'\t%m='08'\t%Om='08'\t%d='31'\t%e='31'\t%Od='三十一'\t%Oe='三十一'\n"),
+        SV("%b='8'\t%B='8月'\t%h='8'\t%m='08'\t%Om='08'\t%d='31'\t%e='31'\t%Od='31'\t%Oe='31'\n"),
         lfmt,
         std::chrono::month_day{std::chrono::August, std::chrono::day{31}});
   check(loc,
-        SV("%b='9'\t%B='9月'\t%h='9'\t%m='09'\t%Om='09'\t%d='32'\t%e='32'\t%Od='三十二'\t%Oe='三十二'\n"),
+        SV("%b='9'\t%B='9月'\t%h='9'\t%m='09'\t%Om='09'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
         lfmt,
         std::chrono::month_day{std::chrono::September, std::chrono::day{32}});
   check(loc,
-        SV("%b='10'\t%B='10月'\t%h='10'\t%m='10'\t%Om='10'\t%d='99'\t%e='99'\t%Od='九十九'\t%Oe='九十九'\n"),
+        SV("%b='10'\t%B='10月'\t%h='10'\t%m='10'\t%Om='10'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
         lfmt,
         std::chrono::month_day{std::chrono::October, std::chrono::day{99}});
   check(loc,
-        SV("%b='11'\t%B='11月'\t%h='11'\t%m='11'\t%Om='11'\t%d='100'\t%e='100'\t%Od='100'\t%Oe='100'\n"),
+        SV("%b='11'\t%B='11月'\t%h='11'\t%m='11'\t%Om='11'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
         lfmt,
         std::chrono::month_day{std::chrono::November, std::chrono::day{100}});
   check(loc,
-        SV("%b='12'\t%B='12月'\t%h='12'\t%m='12'\t%Om='12'\t%d='255'\t%e='255'\t%Od='255'\t%Oe='255'\n"),
+        SV("%b='12'\t%B='12月'\t%h='12'\t%m='12'\t%Om='12'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
         lfmt,
         std::chrono::month_day{std::chrono::December, std::chrono::day{255}});
 #elif defined(_AIX)      // defined(_WIN32)
diff --git a/libcxx/test/std/time/time.syn/formatter.sys_time.pass.cpp b/libcxx/test/std/time/time.syn/formatter.sys_time.pass.cpp
--- a/libcxx/test/std/time/time.syn/formatter.sys_time.pass.cpp
+++ b/libcxx/test/std/time/time.syn/formatter.sys_time.pass.cpp
@@ -14,9 +14,6 @@
 // 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
-
 // REQUIRES: locale.fr_FR.UTF-8
 // REQUIRES: locale.ja_JP.UTF-8
 
@@ -240,7 +237,7 @@
         lfmt,
         std::chrono::sys_seconds(1'234'567'890s)); // 23:31:30 UTC on Friday, 13 February 2009
 
-#endif                                             // _WIN32
+#endif // _WIN32
 
   std::locale::global(std::locale::classic());
 }
@@ -651,8 +648,8 @@
         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)
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("%H='00'\t"
            "%OH='00'\t"
@@ -673,6 +670,10 @@
            "%r='12:00:00 AM'\t"
            "%X='00時00分00秒'\t"
            "%EX='00時00分00秒'\t"
+#  elif defined(_WIN32)
+           "%r='0:00:00'\t"
+           "%X='0:00:00'\t"
+           "%EX='0:00:00'\t"
 #  else
            "%r='午前12:00:00'\t"
            "%X='00:00:00'\t"
@@ -702,6 +703,10 @@
            "%r='11:31:30 PM'\t"
            "%X='23時31分30秒'\t"
            "%EX='23時31分30秒'\t"
+#  elif defined(_WIN32)
+           "%r='23:31:30'\t"
+           "%X='23:31:30'\t"
+           "%EX='23:31:30'\t"
 #  else
            "%r='午後11:31:30'\t"
            "%X='23:31:30'\t"
@@ -710,7 +715,7 @@
            "\n"),
         lfmt,
         std::chrono::hh_mm_ss(23h + 31min + 30s + 123ms));
-#else  // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
   check(loc,
         SV("%H='00'\t"
            "%OH='〇'\t"
@@ -749,7 +754,7 @@
         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)
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
 
   std::locale::global(std::locale::classic());
 }
@@ -782,6 +787,8 @@
       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"),
+#elif defined(_WIN32)
+      SV("%c='01/01/1970 00:00:00'\t%Ec='01/01/1970 00:00:00'\n"),
 #else
       SV("%c='jeu. 01 janv. 1970 00:00:00'\t%Ec='jeu. 01 janv. 1970 00:00:00'\n"),
 #endif
@@ -796,6 +803,8 @@
       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"),
+#elif defined(_WIN32)
+      SV("%c='13/02/2009 23:31:30'\t%Ec='13/02/2009 23:31:30'\n"),
 #else
       SV("%c='ven. 13 févr. 2009 23:31:30'\t%Ec='ven. 13 févr. 2009 23:31:30'\n"),
 #endif
@@ -821,6 +830,15 @@
         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
+#elif defined(_WIN32)                              // __APPLE__
+  check(loc,
+        SV("%c='1970/01/01 0:00:00'\t%Ec='1970/01/01 0:00:00'\n"),
+        lfmt,
+        std::chrono::sys_seconds(0s)); // 00:00:00 UTC Thursday, 1 January 1970
+  check(loc,
+        SV("%c='2009/02/13 23:31:30'\t%Ec='2009/02/13 23:31:30'\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"),
@@ -866,6 +884,22 @@
         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
+#  elif defined(_WIN32)                // 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='-0000'\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"),
@@ -884,7 +918,7 @@
         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__)
+#endif // !defined(__APPLE__)
 }
 
 template <class CharT>
diff --git a/libcxx/test/std/time/time.syn/formatter.weekday.pass.cpp b/libcxx/test/std/time/time.syn/formatter.weekday.pass.cpp
--- a/libcxx/test/std/time/time.syn/formatter.weekday.pass.cpp
+++ b/libcxx/test/std/time/time.syn/formatter.weekday.pass.cpp
@@ -11,12 +11,12 @@
 // UNSUPPORTED: no-localization
 // UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
 
-// TODO FMT Investigate Windows issues.
-// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
-
 // TODO FMT This test should not require std::to_chars(floating-point)
 // XFAIL: availability-fp_to_chars-missing
 
+// TODO FMT Investigate Windows issues.
+// XFAIL: msvc
+
 // REQUIRES: locale.fr_FR.UTF-8
 // REQUIRES: locale.ja_JP.UTF-8
 
@@ -142,7 +142,7 @@
   // Use supplied locale (ja_JP). This locale has a different alternate.
   check(loc, SV("%u='8'\t%Ou='8'\t%w='8'\t%Ow='8'\n"), lfmt, std::chrono::weekday(8));
   check(loc, SV("%u='255'\t%Ou='255'\t%w='255'\t%Ow='255'\n"), lfmt, std::chrono::weekday(255));
-#elif defined(_AIX)
+#elif defined(_AIX)   // defined(__APPLE__)
   // Non localized output using C-locale
   check(SV("%u='8'\t%Ou='8'\t%w='8'\t%Ow='8'\n"), fmt, std::chrono::weekday(8));
   check(SV("%u='5'\t%Ou='5'\t%w='5'\t%Ow='5'\n"), fmt, std::chrono::weekday(255));
@@ -154,7 +154,19 @@
   // Use supplied locale (ja_JP). This locale has a different alternate.
   check(loc, SV("%u='8'\t%Ou='8'\t%w='8'\t%Ow='8'\n"), lfmt, std::chrono::weekday(8));
   check(loc, SV("%u='5'\t%Ou='5'\t%w='5'\t%Ow='5'\n"), lfmt, std::chrono::weekday(255));
-#else
+#elif defined(_WIN32) // defined(__APPLE__)
+  // Non localized output using C-locale
+  check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), fmt, std::chrono::weekday(8));
+  check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), fmt, std::chrono::weekday(255));
+
+  // Use the global locale (fr_FR)
+  check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday(8));
+  check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday(255));
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+  check(loc, SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday(8));
+  check(loc, SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday(255));
+#else                 // defined(__APPLE__)
   // Non localized output using C-locale
   check(SV("%u='1'\t%Ou='1'\t%w='8'\t%Ow='8'\n"), fmt, std::chrono::weekday(8));
   check(SV("%u='3'\t%Ou='3'\t%w='255'\t%Ow='255'\n"), fmt, std::chrono::weekday(255));
@@ -166,8 +178,7 @@
   // Use supplied locale (ja_JP). This locale has a different alternate.
   check(loc, SV("%u='1'\t%Ou='一'\t%w='8'\t%Ow='八'\n"), lfmt, std::chrono::weekday(8));
   check(loc, SV("%u='3'\t%Ou='三'\t%w='255'\t%Ow='255'\n"), lfmt, std::chrono::weekday(255));
-
-#endif
+#endif                // defined(__APPLE__)
 
   std::locale::global(std::locale::classic());
 }
diff --git a/libcxx/test/std/time/time.syn/formatter.weekday_index.pass.cpp b/libcxx/test/std/time/time.syn/formatter.weekday_index.pass.cpp
--- a/libcxx/test/std/time/time.syn/formatter.weekday_index.pass.cpp
+++ b/libcxx/test/std/time/time.syn/formatter.weekday_index.pass.cpp
@@ -11,12 +11,12 @@
 // UNSUPPORTED: no-localization
 // UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
 
-// TODO FMT Investigate Windows issues.
-// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
-
 // TODO FMT This test should not require std::to_chars(floating-point)
 // XFAIL: availability-fp_to_chars-missing
 
+// TODO FMT Investigate Windows issues.
+// XFAIL: msvc
+
 // REQUIRES: locale.fr_FR.UTF-8
 // REQUIRES: locale.ja_JP.UTF-8
 
@@ -284,6 +284,24 @@
           SV("%u='255'\t%Ou='255'\t%w='255'\t%Ow='255'\n"),
           lfmt,
           std::chrono::weekday_indexed{std::chrono::weekday(255), 1});
+#elif defined(_WIN32) //  defined(__APPLE__)
+    // Non localized output using C-locale
+    check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), fmt, std::chrono::weekday_indexed{std::chrono::weekday(8), 0});
+    check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), fmt, std::chrono::weekday_indexed{std::chrono::weekday(8), 1});
+    check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), fmt, std::chrono::weekday_indexed{std::chrono::weekday(255), 0});
+    check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), fmt, std::chrono::weekday_indexed{std::chrono::weekday(255), 1});
+
+    // Use the global locale (fr_FR)
+    check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_indexed{std::chrono::weekday(8), 0});
+    check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_indexed{std::chrono::weekday(8), 1});
+    check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_indexed{std::chrono::weekday(255), 0});
+    check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_indexed{std::chrono::weekday(255), 1});
+
+    // Use supplied locale (ja_JP). This locale has a different alternate.
+    check(loc, SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_indexed{std::chrono::weekday(8), 0});
+    check(loc, SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_indexed{std::chrono::weekday(8), 1});
+    check(loc, SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_indexed{std::chrono::weekday(255), 0});
+    check(loc, SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_indexed{std::chrono::weekday(255), 1});
 #elif defined(_AIX) //  defined(__APPLE__)
     // Non localized output using C-locale
     check(SV("%u='8'\t%Ou='8'\t%w='8'\t%Ow='8'\n"), fmt, std::chrono::weekday_indexed{std::chrono::weekday(8), 0});
diff --git a/libcxx/test/std/time/time.syn/formatter.weekday_last.pass.cpp b/libcxx/test/std/time/time.syn/formatter.weekday_last.pass.cpp
--- a/libcxx/test/std/time/time.syn/formatter.weekday_last.pass.cpp
+++ b/libcxx/test/std/time/time.syn/formatter.weekday_last.pass.cpp
@@ -11,12 +11,12 @@
 // UNSUPPORTED: no-localization
 // UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
 
-// TODO FMT Investigate Windows issues.
-// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
-
 // TODO FMT This test should not require std::to_chars(floating-point)
 // XFAIL: availability-fp_to_chars-missing
 
+// TODO FMT Investigate Windows issues.
+// XFAIL: msvc
+
 // REQUIRES: locale.fr_FR.UTF-8
 // REQUIRES: locale.ja_JP.UTF-8
 
@@ -248,6 +248,18 @@
         SV("%u='255'\t%Ou='255'\t%w='255'\t%Ow='255'\n"),
         lfmt,
         std::chrono::weekday_last{std::chrono::weekday(255)});
+#elif defined(_WIN32) // defined(__APPLE__)
+  // Non localized output using C-locale
+  check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), fmt, std::chrono::weekday_last{std::chrono::weekday(8)});
+  check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), fmt, std::chrono::weekday_last{std::chrono::weekday(255)});
+
+  // Use the global locale (fr_FR)
+  check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_last{std::chrono::weekday(8)});
+  check(SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_last{std::chrono::weekday(255)});
+
+  // Use supplied locale (ja_JP). This locale has a different alternate.
+  check(loc, SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_last{std::chrono::weekday(8)});
+  check(loc, SV("%u=''\t%Ou=''\t%w=''\t%Ow=''\n"), lfmt, std::chrono::weekday_last{std::chrono::weekday(255)});
 #elif defined(_AIX) // defined(__APPLE__)
   // Non localized output using C-locale
   check(SV("%u='8'\t%Ou='8'\t%w='8'\t%Ow='8'\n"), fmt, std::chrono::weekday_last{std::chrono::weekday(8)});
diff --git a/libcxx/test/std/time/time.syn/formatter.year_month_day.pass.cpp b/libcxx/test/std/time/time.syn/formatter.year_month_day.pass.cpp
--- a/libcxx/test/std/time/time.syn/formatter.year_month_day.pass.cpp
+++ b/libcxx/test/std/time/time.syn/formatter.year_month_day.pass.cpp
@@ -11,12 +11,12 @@
 // UNSUPPORTED: no-localization
 // UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
 
-// TODO FMT Investigate Windows issues.
-// UNSUPPORTED: msvc, target={{.+}}-windows-gnu
-
 // TODO FMT This test should not require std::to_chars(floating-point)
 // XFAIL: availability-fp_to_chars-missing
 
+// TODO FMT Investigate Windows issues.
+// XFAIL: msvc
+
 // REQUIRES: locale.fr_FR.UTF-8
 // REQUIRES: locale.ja_JP.UTF-8
 
@@ -64,6 +64,17 @@
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::month{2}, std::chrono::day{31}});
 
   // Valid year, invalid month, valid day
+#ifdef _WIN32
+  check(SV(" is not a valid date"),
+        SV("{}"),
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::month{0}, std::chrono::day{31}});
+  check(SV("****** is not a valid date******"),
+        SV("{:*^32}"),
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::month{0}, std::chrono::day{31}});
+  check(SV("*********** is not a valid date"),
+        SV("{:*>31}"),
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::month{0}, std::chrono::day{31}});
+#else  // _WIN32
   check(SV("1970-00-31 is not a valid date"),
         SV("{}"),
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::month{0}, std::chrono::day{31}});
@@ -73,8 +84,20 @@
   check(SV("*1970-00-31 is not a valid date"),
         SV("{:*>31}"),
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::month{0}, std::chrono::day{31}});
+#endif // _WIN32
 
   // Valid year, invalid month, invalid day
+#ifdef _WIN32
+  check(SV(" is not a valid date"),
+        SV("{}"),
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::month{0}, std::chrono::day{32}});
+  check(SV("****** is not a valid date******"),
+        SV("{:*^32}"),
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::month{0}, std::chrono::day{32}});
+  check(SV("*********** is not a valid date"),
+        SV("{:*>31}"),
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::month{0}, std::chrono::day{32}});
+#else  // _WIN32
   check(SV("1970-00-32 is not a valid date"),
         SV("{}"),
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::month{0}, std::chrono::day{32}});
@@ -84,6 +107,7 @@
   check(SV("*1970-00-32 is not a valid date"),
         SV("{:*>31}"),
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::month{0}, std::chrono::day{32}});
+#endif // _WIN32
 
   // Invalid year, valid month, valid day
   check(SV("-32768-01-31 is not a valid date"),
@@ -397,9 +421,15 @@
   std::locale::global(std::locale(LOCALE_fr_FR_UTF_8));
 
   // Non localized output using C-locale
+#ifdef _WIN32
+  check(SV("%b='Jan'\t%B='January'\t%h='Jan'\t%m='01'\t%Om='01'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        fmt,
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::January, std::chrono::day{0}});
+#else
   check(SV("%b='Jan'\t%B='January'\t%h='Jan'\t%m='01'\t%Om='01'\t%d='00'\t%e=' 0'\t%Od='00'\t%Oe=' 0'\n"),
         fmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::January, std::chrono::day{0}});
+#endif
   check(SV("%b='Feb'\t%B='February'\t%h='Feb'\t%m='02'\t%Om='02'\t%d='01'\t%e=' 1'\t%Od='01'\t%Oe=' 1'\n"),
         fmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::February, std::chrono::day{1}});
@@ -421,27 +451,42 @@
   check(SV("%b='Aug'\t%B='August'\t%h='Aug'\t%m='08'\t%Om='08'\t%d='31'\t%e='31'\t%Od='31'\t%Oe='31'\n"),
         fmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::August, std::chrono::day{31}});
+#ifdef _WIN32
+  check(SV("%b='Sep'\t%B='September'\t%h='Sep'\t%m='09'\t%Om='09'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        fmt,
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::September, std::chrono::day{32}});
+  check(SV("%b='Oct'\t%B='October'\t%h='Oct'\t%m='10'\t%Om='10'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        fmt,
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::October, std::chrono::day{99}});
+  check(SV("%b='Nov'\t%B='November'\t%h='Nov'\t%m='11'\t%Om='11'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        fmt,
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::November, std::chrono::day{100}});
+  check(SV("%b='Dec'\t%B='December'\t%h='Dec'\t%m='12'\t%Om='12'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        fmt,
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::December, std::chrono::day{255}});
+#else // _WIN32
   check(SV("%b='Sep'\t%B='September'\t%h='Sep'\t%m='09'\t%Om='09'\t%d='32'\t%e='32'\t%Od='32'\t%Oe='32'\n"),
         fmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::September, std::chrono::day{32}});
   check(SV("%b='Oct'\t%B='October'\t%h='Oct'\t%m='10'\t%Om='10'\t%d='99'\t%e='99'\t%Od='99'\t%Oe='99'\n"),
         fmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::October, std::chrono::day{99}});
-#if defined(_AIX)
+#  if defined(_AIX)
   check(SV("%b='Nov'\t%B='November'\t%h='Nov'\t%m='11'\t%Om='11'\t%d='00'\t%e=' 0'\t%Od='00'\t%Oe=' 0'\n"),
         fmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::November, std::chrono::day{100}});
   check(SV("%b='Dec'\t%B='December'\t%h='Dec'\t%m='12'\t%Om='12'\t%d='55'\t%e='55'\t%Od='55'\t%Oe='55'\n"),
         fmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::December, std::chrono::day{255}});
-#else  //  defined(_AIX)
+#  else  //  defined(_AIX)
   check(SV("%b='Nov'\t%B='November'\t%h='Nov'\t%m='11'\t%Om='11'\t%d='100'\t%e='100'\t%Od='100'\t%Oe='100'\n"),
         fmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::November, std::chrono::day{100}});
   check(SV("%b='Dec'\t%B='December'\t%h='Dec'\t%m='12'\t%Om='12'\t%d='255'\t%e='255'\t%Od='255'\t%Oe='255'\n"),
         fmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::December, std::chrono::day{255}});
-#endif //  defined(_AIX)
+#  endif //  defined(_AIX)
+#endif   // _WIN32
 
   // Use the global locale (fr_FR)
 #if defined(__APPLE__)
@@ -481,10 +526,16 @@
   check(SV("%b='déc'\t%B='décembre'\t%h='déc'\t%m='12'\t%Om='12'\t%d='255'\t%e='255'\t%Od='255'\t%Oe='255'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::December, std::chrono::day{255}});
-#else    // defined(__APPLE__)
+#else // defined(__APPLE__)
+#  ifdef _WIN32
+  check(SV("%b='janv.'\t%B='janvier'\t%h='janv.'\t%m='01'\t%Om='01'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        lfmt,
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::January, std::chrono::day{0}});
+#  else
   check(SV("%b='janv.'\t%B='janvier'\t%h='janv.'\t%m='01'\t%Om='01'\t%d='00'\t%e=' 0'\t%Od='00'\t%Oe=' 0'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::January, std::chrono::day{0}});
+#  endif
   check(SV("%b='févr.'\t%B='février'\t%h='févr.'\t%m='02'\t%Om='02'\t%d='01'\t%e=' 1'\t%Od='01'\t%Oe=' 1'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::February, std::chrono::day{1}});
@@ -511,77 +562,92 @@
   check(SV("%b='août'\t%B='août'\t%h='août'\t%m='08'\t%Om='08'\t%d='31'\t%e='31'\t%Od='31'\t%Oe='31'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::August, std::chrono::day{31}});
+#  ifdef _WIN32
+  check(SV("%b='sept.'\t%B='septembre'\t%h='sept.'\t%m='09'\t%Om='09'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        lfmt,
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::September, std::chrono::day{32}});
+  check(SV("%b='oct.'\t%B='octobre'\t%h='oct.'\t%m='10'\t%Om='10'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        lfmt,
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::October, std::chrono::day{99}});
+  check(SV("%b='nov.'\t%B='novembre'\t%h='nov.'\t%m='11'\t%Om='11'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        lfmt,
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::November, std::chrono::day{100}});
+  check(SV("%b='déc.'\t%B='décembre'\t%h='déc.'\t%m='12'\t%Om='12'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
+        lfmt,
+        std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::December, std::chrono::day{255}});
+#  else // _WIN32
   check(SV("%b='sept.'\t%B='septembre'\t%h='sept.'\t%m='09'\t%Om='09'\t%d='32'\t%e='32'\t%Od='32'\t%Oe='32'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::September, std::chrono::day{32}});
   check(SV("%b='oct.'\t%B='octobre'\t%h='oct.'\t%m='10'\t%Om='10'\t%d='99'\t%e='99'\t%Od='99'\t%Oe='99'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::October, std::chrono::day{99}});
-#  if defined(_AIX)
+#    if defined(_AIX)
   check(SV("%b='nov.'\t%B='novembre'\t%h='nov.'\t%m='11'\t%Om='11'\t%d='00'\t%e=' 0'\t%Od='00'\t%Oe=' 0'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::November, std::chrono::day{100}});
   check(SV("%b='déc.'\t%B='décembre'\t%h='déc.'\t%m='12'\t%Om='12'\t%d='55'\t%e='55'\t%Od='55'\t%Oe='55'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::December, std::chrono::day{255}});
-#  else  //   defined(_AIX)
+#    else  //   defined(_AIX)
   check(SV("%b='nov.'\t%B='novembre'\t%h='nov.'\t%m='11'\t%Om='11'\t%d='100'\t%e='100'\t%Od='100'\t%Oe='100'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::November, std::chrono::day{100}});
   check(SV("%b='déc.'\t%B='décembre'\t%h='déc.'\t%m='12'\t%Om='12'\t%d='255'\t%e='255'\t%Od='255'\t%Oe='255'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::December, std::chrono::day{255}});
-#  endif //   defined(_AIX)
-#endif   // defined(__APPLE__)
+#    endif //   defined(_AIX)
+#  endif   // _WIN32
+#endif     // defined(__APPLE__)
 
   // Use supplied locale (ja_JP)
 #if defined(_WIN32)
   check(loc,
-        SV("%b='1'\t%B='1月'\t%h='1'\t%m='01'\t%Om='01'\t%d='00'\t%e=' 0'\t%Od='〇'\t%Oe='〇'\n"),
+        SV("%b='1'\t%B='1月'\t%h='1'\t%m='01'\t%Om='01'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::January, std::chrono::day{0}});
   check(loc,
-        SV("%b='2'\t%B='2月'\t%h='2'\t%m='02'\t%Om='02'\t%d='01'\t%e=' 1'\t%Od='一'\t%Oe='一'\n"),
+        SV("%b='2'\t%B='2月'\t%h='2'\t%m='02'\t%Om='02'\t%d='01'\t%e=' 1'\t%Od='01'\t%Oe=' 1'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::February, std::chrono::day{1}});
   check(loc,
-        SV("%b='3'\t%B='3月'\t%h='3'\t%m='03'\t%Om='03'\t%d='09'\t%e=' 9'\t%Od='九'\t%Oe='九'\n"),
+        SV("%b='3'\t%B='3月'\t%h='3'\t%m='03'\t%Om='03'\t%d='09'\t%e=' 9'\t%Od='09'\t%Oe=' 9'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::March, std::chrono::day{9}});
   check(loc,
-        SV("%b='4'\t%B='4月'\t%h='4'\t%m='04'\t%Om='04'\t%d='10'\t%e='10'\t%Od='十'\t%Oe='十'\n"),
+        SV("%b='4'\t%B='4月'\t%h='4'\t%m='04'\t%Om='04'\t%d='10'\t%e='10'\t%Od='10'\t%Oe='10'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::April, std::chrono::day{10}});
   check(loc,
-        SV("%b='5'\t%B='5月'\t%h='5'\t%m='05'\t%Om='05'\t%d='28'\t%e='28'\t%Od='二十八'\t%Oe='二十八'\n"),
+        SV("%b='5'\t%B='5月'\t%h='5'\t%m='05'\t%Om='05'\t%d='28'\t%e='28'\t%Od='28'\t%Oe='28'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::May, std::chrono::day{28}});
   check(loc,
-        SV("%b='6'\t%B='6月'\t%h='6'\t%m='06'\t%Om='06'\t%d='29'\t%e='29'\t%Od='二十九'\t%Oe='二十九'\n"),
+        SV("%b='6'\t%B='6月'\t%h='6'\t%m='06'\t%Om='06'\t%d='29'\t%e='29'\t%Od='29'\t%Oe='29'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::June, std::chrono::day{29}});
   check(loc,
-        SV("%b='7'\t%B='7月'\t%h='7'\t%m='07'\t%Om='07'\t%d='30'\t%e='30'\t%Od='三十'\t%Oe='三十'\n"),
+        SV("%b='7'\t%B='7月'\t%h='7'\t%m='07'\t%Om='07'\t%d='30'\t%e='30'\t%Od='30'\t%Oe='30'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::July, std::chrono::day{30}});
   check(loc,
-        SV("%b='8'\t%B='8月'\t%h='8'\t%m='08'\t%Om='08'\t%d='31'\t%e='31'\t%Od='三十一'\t%Oe='三十一'\n"),
+        SV("%b='8'\t%B='8月'\t%h='8'\t%m='08'\t%Om='08'\t%d='31'\t%e='31'\t%Od='31'\t%Oe='31'\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::August, std::chrono::day{31}});
   check(loc,
-        SV("%b='9'\t%B='9月'\t%h='9'\t%m='09'\t%Om='09'\t%d='32'\t%e='32'\t%Od='三十二'\t%Oe='三十二'\n"),
+        SV("%b='9'\t%B='9月'\t%h='9'\t%m='09'\t%Om='09'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::September, std::chrono::day{32}});
   check(loc,
-        SV("%b='10'\t%B='10月'\t%h='10'\t%m='10'\t%Om='10'\t%d='99'\t%e='99'\t%Od='九十九'\t%Oe='九十九'\n"),
+        SV("%b='10'\t%B='10月'\t%h='10'\t%m='10'\t%Om='10'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::October, std::chrono::day{99}});
   check(loc,
-        SV("%b='11'\t%B='11月'\t%h='11'\t%m='11'\t%Om='11'\t%d='100'\t%e='100'\t%Od='100'\t%Oe='100'\n"),
+        SV("%b='11'\t%B='11月'\t%h='11'\t%m='11'\t%Om='11'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::November, std::chrono::day{100}});
   check(loc,
-        SV("%b='12'\t%B='12月'\t%h='12'\t%m='12'\t%Om='12'\t%d='255'\t%e='255'\t%Od='255'\t%Oe='255'\n"),
+        SV("%b='12'\t%B='12月'\t%h='12'\t%m='12'\t%Om='12'\t%d=''\t%e=''\t%Od=''\t%Oe=''\n"),
         lfmt,
         std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::December, std::chrono::day{255}});
 #elif defined(_AIX)      // defined(_WIN32)
@@ -945,14 +1011,14 @@
          "%V='01'\t"
          "%w='4'\t"
          "%W='00'\t"
-#if defined(__APPLE__) || defined(_AIX)
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "%x='1970/01/01'\t"
-#else  // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "%x='1970年01月01日'\t"
-#endif // defined(__APPLE__) || defined(_AIX)
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "%y='70'\t"
          "%Y='1970'\t"
-#if defined(__APPLE__) || defined(_AIX)
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "%Ex='1970/01/01'\t"
          "%EC='19'\t"
          "%Ey='70'\t"
@@ -963,7 +1029,7 @@
          "%Ow='4'\t"
          "%OW='00'\t"
          "%Oy='70'\t"
-#else  // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "%Ex='昭和45年01月01日'\t"
          "%EC='昭和'\t"
          "%Ey='45'\t"
@@ -974,7 +1040,7 @@
          "%Ow='四'\t"
          "%OW='〇'\t"
          "%Oy='七十'\t"
-#endif // defined(__APPLE__) || defined(_AIX)
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "\n"),
       lfmt,
       std::chrono::year_month_day{std::chrono::year{1970}, std::chrono::January, std::chrono::day{1}});
@@ -992,14 +1058,14 @@
          "%V='22'\t"
          "%w='6'\t"
          "%W='21'\t"
-#if defined(__APPLE__) || defined(_AIX)
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "%x='2004/05/29'\t"
-#else  // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "%x='2004年05月29日'\t"
-#endif // defined(__APPLE__) || defined(_AIX)
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "%y='04'\t"
          "%Y='2004'\t"
-#if defined(__APPLE__) || defined(_AIX)
+#if defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "%Ex='2004/05/29'\t"
          "%EC='20'\t"
          "%Ey='04'\t"
@@ -1010,7 +1076,7 @@
          "%Ow='6'\t"
          "%OW='21'\t"
          "%Oy='04'\t"
-#else  // defined(__APPLE__) || defined(_AIX)
+#else  // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "%Ex='平成16年05月29日'\t"
          "%EC='平成'\t"
          "%Ey='16'\t"
@@ -1021,7 +1087,7 @@
          "%Ow='六'\t"
          "%OW='二十一'\t"
          "%Oy='四'\t"
-#endif // defined(__APPLE__) || defined(_AIX)
+#endif // defined(__APPLE__) || defined(_AIX) || defined(_WIN32)
          "\n"),
       lfmt,
       std::chrono::year_month_day{std::chrono::year{2004}, std::chrono::May, std::chrono::day{29}});
diff --git a/libcxx/test/std/time/time.syn/formatter_tests.h b/libcxx/test/std/time/time.syn/formatter_tests.h
--- a/libcxx/test/std/time/time.syn/formatter_tests.h
+++ b/libcxx/test/std/time/time.syn/formatter_tests.h
@@ -8,6 +8,8 @@
 #ifndef TEST_STD_TIME_TIME_SYN_FORMATTER_TESTS_H
 #define TEST_STD_TIME_TIME_SYN_FORMATTER_TESTS_H
 
+#include "assert_macros.h"
+#include "concat_macros.h"
 #include "make_string.h"
 #include "string_literal.h"
 #include "test_format_string.h"
@@ -34,11 +36,9 @@
 template <class CharT, class... Args>
 void check(std::basic_string_view<CharT> expected, test_format_string<CharT, Args...> fmt, Args&&... args) {
   std::basic_string<CharT> out = std::format(fmt, std::forward<Args>(args)...);
-  if constexpr (std::same_as<CharT, char>)
-    if (out != expected)
-      std::cerr << "\nFormat string   " << fmt.get() << "\nExpected output " << expected << "\nActual output   " << out
-                << '\n';
-  assert(out == expected);
+  TEST_REQUIRE(out == expected,
+               TEST_WRITE_CONCATENATED(
+                   "\nFormat string   ", fmt.get(), "\nExpected output ", expected, "\nActual output   ", out, '\n'));
 }
 
 template <class CharT, class... Args>
@@ -47,38 +47,24 @@
            test_format_string<CharT, Args...> fmt,
            Args&&... args) {
   std::basic_string<CharT> out = std::format(loc, fmt, std::forward<Args>(args)...);
-  if constexpr (std::same_as<CharT, char>)
-    if (out != expected)
-      std::cerr << "\nFormat string   " << fmt.get() << "\nExpected output " << expected << "\nActual output   " << out
-                << '\n';
-  assert(out == expected);
+  TEST_REQUIRE(out == expected,
+               TEST_WRITE_CONCATENATED(
+                   "\nFormat string   ", fmt.get(), "\nExpected output ", expected, "\nActual output   ", out, '\n'));
 }
 
 template <class CharT, class... Args>
 void check_exception([[maybe_unused]] std::string_view what,
                      [[maybe_unused]] std::basic_string_view<CharT> fmt,
                      [[maybe_unused]] const Args&... args) {
-#ifndef TEST_HAS_NO_EXCEPTIONS
-  try {
-    TEST_IGNORE_NODISCARD std::vformat(fmt, std::make_format_args<format_context<CharT>>(args...));
-    if constexpr (std::same_as<CharT, char>)
-      std::cerr << "\nFormat string   " << fmt << "\nDidn't throw an exception.\n";
-    assert(false);
-  } catch (const std::format_error& e) {
-#  if defined(_LIBCPP_VERSION)
-    if constexpr (std::same_as<CharT, char>)
-      if (e.what() != what)
-        std::cerr << "\nFormat string   " << fmt << "\nExpected exception " << what << "\nActual exception   "
-                  << e.what() << '\n';
-    assert(e.what() == what);
-#  else
-    (void)what;
-    (void)e;
-#  endif
-    return;
-  }
-  assert(false);
-#endif
+  TEST_VALIDATE_EXCEPTION(
+      std::format_error,
+      [&]([[maybe_unused]] const std::format_error& e) {
+        TEST_LIBCPP_REQUIRE(
+            e.what() == what,
+            TEST_WRITE_CONCATENATED(
+                "\nFormat string   ", fmt, "\nExpected exception ", what, "\nActual exception   ", e.what(), '\n'));
+      },
+      TEST_IGNORE_NODISCARD std::vformat(fmt, std::make_format_args<format_context<CharT>>(args...)));
 }
 
 template <class CharT, class T>
diff --git a/libcxx/test/support/concat_macros.h b/libcxx/test/support/concat_macros.h
--- a/libcxx/test/support/concat_macros.h
+++ b/libcxx/test/support/concat_macros.h
@@ -16,15 +16,153 @@
 #include "test_macros.h"
 
 #ifndef TEST_HAS_NO_LOCALIZATION
+#  include <concepts>
+#  include <iterator>
 #  include <sstream>
 #endif
 
 #if TEST_STD_VER > 17
 
 #  ifndef TEST_HAS_NO_LOCALIZATION
+
+[[nodiscard]] constexpr bool test_is_high_surrogate(char32_t value) { return value >= 0xd800 && value <= 0xdbff; }
+
+[[nodiscard]] constexpr bool test_is_low_surrogate(char32_t value) { return value >= 0xdc00 && value <= 0xdfff; }
+
+[[nodiscard]] constexpr bool test_is_surrogate(char32_t value) { return value >= 0xd800 && value <= 0xdfff; }
+
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr bool test_is_code_point(char32_t value) { return value <= 0x10ffff; }
+
+[[nodiscard]] constexpr bool test_is_scalar_value(char32_t value) {
+  return test_is_code_point(value) && !test_is_surrogate(value);
+}
+
+inline constexpr char32_t test_replacement_character = U'\ufffd';
+
+template <class InIt, class OutIt>
+OutIt test_transcode() = delete;
+
+template <class InIt, class OutIt>
+  requires(std::output_iterator<OutIt, const char&> && std::same_as<std::iter_value_t<InIt>, char8_t>)
+OutIt test_transcode(InIt first, InIt last, OutIt out_it) {
+  return std::copy(first, last, out_it);
+}
+
+template <class OutIt>
+  requires std::output_iterator<OutIt, const char&>
+void test_encode(OutIt& out_it, char16_t value) {
+  if (value < 0x80)
+    *out_it++ = value;
+  else if (value < 0x800) {
+    *out_it++ = 0b1100'0000 | (value >> 6);
+    *out_it++ = 0b1000'0000 | (value & 0b0011'1111);
+  } else {
+    *out_it++ = 0b1110'0000 | (value >> 12);
+    *out_it++ = 0b1000'0000 | ((value) >> 6 & 0b0011'1111);
+    *out_it++ = 0b1000'0000 | (value & 0b0011'1111);
+  }
+}
+
+template <class OutIt>
+  requires std::output_iterator<OutIt, const char&>
+void test_encode(OutIt& out_it, char32_t value) {
+  if ((value & 0xffff0000) == 0)
+    test_encode(out_it, static_cast<char16_t>(value));
+  else {
+    *out_it++ = 0b1110'0000 | (value >> 18);
+    *out_it++ = 0b1000'0000 | ((value) >> 12 & 0b0011'1111);
+    *out_it++ = 0b1000'0000 | ((value) >> 6 & 0b0011'1111);
+    *out_it++ = 0b1000'0000 | (value & 0b0011'1111);
+  }
+}
+
+template <class InIt, class OutIt>
+  requires(std::output_iterator<OutIt, const char&> &&
+           (std::same_as<std::iter_value_t<InIt>, char16_t>
+#    ifndef TEST_HAS_NO_WIDE_CHARACTERS
+            || (std::same_as<std::iter_value_t<InIt>, wchar_t> && sizeof(wchar_t) == 2))
+#    endif
+               )
+OutIt test_transcode(InIt first, InIt last, OutIt out_it) {
+  while (first != last) {
+    char32_t value = *first++;
+
+    if (test_is_low_surrogate(value)) [[unlikely]] {
+      test_encode(out_it, static_cast<char16_t>(test_replacement_character));
+      continue;
+    }
+
+    if (!test_is_high_surrogate(value)) {
+      test_encode(out_it, static_cast<char16_t>(value));
+      continue;
+    }
+
+    if (first == last || !test_is_low_surrogate(static_cast<char32_t>(*first))) [[unlikely]] {
+      test_encode(out_it, static_cast<char16_t>(test_replacement_character));
+      continue;
+    }
+
+    value -= 0xd800;
+    value <<= 10;
+    value += static_cast<char32_t>(*first++) - 0xdc00;
+    value += 0x10000;
+
+    if (test_is_code_point(value)) [[likely]]
+      test_encode(out_it, value);
+    else
+      test_encode(out_it, static_cast<char16_t>(test_replacement_character));
+  }
+
+  return out_it;
+}
+
+template <class InIt, class OutIt>
+  requires(std::output_iterator<OutIt, const char&> &&
+           (std::same_as<std::iter_value_t<InIt>, char32_t> ||
+#    ifndef TEST_HAS_NO_WIDE_CHARACTERS
+            (std::same_as<std::iter_value_t<InIt>, wchar_t> && sizeof(wchar_t) == 4))
+#    endif
+               )
+OutIt test_transcode(InIt first, InIt last, OutIt out_it) {
+  while (first != last) {
+    char32_t value = *first++;
+    if (test_is_code_point(value)) [[likely]]
+      test_encode(out_it, value);
+    else
+      test_encode(out_it, static_cast<char16_t>(test_replacement_character));
+  }
+  return out_it;
+}
+
 template <class T>
-concept test_char_streamable = requires(T&& value) { std::stringstream{} << std::forward<T>(value); };
-#  endif
+concept test_streamable = requires(std::stringstream& stream, T&& value) { stream << value; };
+
+template <class T>
+concept test_convertable = (!test_streamable<T> && requires(T&& value) {
+  std::basic_string_view{std::begin(value), std::end(value)};
+});
+
+template <class T>
+concept test_can_concat = test_streamable<T> || test_convertable<T>;
+
+template <test_streamable T>
+std::ostream& test_concat(std::ostream& stream, T&& value) {
+  return stream << value;
+}
+
+template <test_convertable T>
+std::ostream& test_concat(std::ostream& stream, T&& value) {
+  auto b = std::begin(value);
+  auto e = std::end(value);
+  if (b != e) {
+    // When T is an array it's string-literal, remove the NUL terminator.
+    if constexpr (std::is_array_v<std::remove_cvref_t<T>>)
+      --e;
+    test_transcode(b, e, std::ostream_iterator<char>{stream});
+  }
+  return stream;
+}
+#  endif // TEST_HAS_NO_LOCALIZATION
 
 // If possible concatenates message for the assertion function, else returns a
 // default message. Not being able to stream is not considered and error. For
@@ -37,12 +175,12 @@
 template <class... Args>
 std::string test_concat_message([[maybe_unused]] Args&&... args) {
 #  ifndef TEST_HAS_NO_LOCALIZATION
-  if constexpr ((test_char_streamable<Args> && ...)) {
+  if constexpr ((test_can_concat<Args> && ...)) {
     std::stringstream sstr;
-    ((sstr << std::forward<Args>(args)), ...);
+    ((test_concat(sstr, std::forward<Args>(args))), ...);
     return sstr.str();
   } else
-#  endif
+#  endif // TEST_HAS_NO_LOCALIZATION
     return "Message discarded since it can't be streamed to std::cerr.\n";
 }
 
@@ -50,5 +188,4 @@
 #  define TEST_WRITE_CONCATENATED(...) [&] { ::test_eprintf("%s", ::test_concat_message(__VA_ARGS__).c_str()); }
 
 #endif // TEST_STD_VER > 17
-
 #endif //  TEST_SUPPORT_CONCAT_MACROS_H