From 1f0e6cf0506e4b8570da8d8eb9d22c31b0d93d1d Mon Sep 17 00:00:00 2001
From: fhgffy <102001626+fhgffy@users.noreply.github.com>
Date: Fri, 9 Oct 2026 00:03:03 +0800
Subject: [PATCH] Fix weekday and day of year when formatting year_month_day

The year_month_day formatter only filled in tm_year, tm_mon and tm_mday,
so specifiers that depend on the weekday or the day of the year (%a, %A,
%u, %w, %j, %U, %W, %V, %G, %g) were formatted from zero-initialized
fields, e.g. every date was printed as a Sunday. Compute tm_wday and
tm_yday from the date before passing it to the tm formatter.
---
 include/fmt/chrono.h | 29 +++++++++++++++++++++++++----
 test/chrono-test.cc  | 15 +++++++++++++++
 2 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index 19571164..5a186913 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -926,6 +926,18 @@ inline auto tm_wday_short_name(int wday) -> const char* {
   return wday >= 0 && wday <= 6 ? short_name_list[wday] : "???";
 }
 
+// Returns the number of days from 1970-01-01 to the given date in the
+// proleptic Gregorian calendar.
+inline auto days_from_civil(int y, unsigned m, unsigned d) noexcept
+    -> long long {
+  if (m <= 2) --y;
+  long long era = (y >= 0 ? y : y - 399) / 400;
+  auto yoe = static_cast<unsigned>(y - era * 400);
+  unsigned doy = (153 * (m > 2 ? m - 3 : m + 9) + 2) / 5 + d - 1;
+  unsigned doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
+  return era * 146097 + static_cast<long long>(doe) - 719468;
+}
+
 inline auto tm_mon_full_name(int mon) -> const char* {
   static constexpr const char* full_name_list[] = {
       "January", "February", "March",     "April",   "May",      "June",
@@ -2035,10 +2047,19 @@ struct formatter<year_month_day, Char> : private formatter<std::tm, Char> {
   auto format(year_month_day val, FormatContext& ctx) const
       -> decltype(ctx.out()) {
     auto time = std::tm();
-    time.tm_year = static_cast<int>(val.year()) - 1900;
-    time.tm_mon = static_cast<int>(static_cast<unsigned>(val.month())) - 1;
-    time.tm_mday = static_cast<int>(static_cast<unsigned>(val.day()));
-    if (use_tm_formatter_) return formatter<std::tm, Char>::format(time, ctx);
+    auto y = static_cast<int>(val.year());
+    auto m = static_cast<unsigned>(val.month());
+    auto d = static_cast<unsigned>(val.day());
+    time.tm_year = y - 1900;
+    time.tm_mon = static_cast<int>(m) - 1;
+    time.tm_mday = static_cast<int>(d);
+    if (use_tm_formatter_) {
+      // 1970-01-01 was a Thursday.
+      auto days = detail::days_from_civil(y, m, d);
+      time.tm_wday = static_cast<int>((days % 7 + 11) % 7);
+      time.tm_yday = static_cast<int>(days - detail::days_from_civil(y, 1, 1));
+      return formatter<std::tm, Char>::format(time, ctx);
+    }
     auto w = detail::tm_writer<decltype(ctx.out()), Char>(locale_ref(), false,
                                                           ctx.out(), time);
     w.on_iso_date();
diff --git a/test/chrono-test.cc b/test/chrono-test.cc
index 818ec42f..d2300dfe 100644
--- a/test/chrono-test.cc
+++ b/test/chrono-test.cc
@@ -1044,3 +1044,18 @@ TEST(chrono_test, year_month_day) {
     EXPECT_THAT(months, Contains(fmt::format("{:L}", month)));
   }
 }
+
+TEST(chrono_test, year_month_day_weekday_and_day_of_year) {
+  auto ymd = fmt::year_month_day(fmt::year(2024), fmt::month(3), fmt::day(1));
+  EXPECT_EQ(fmt::format("{:%a %A %u %w}", ymd), "Fri Friday 5 5");
+  EXPECT_EQ(fmt::format("{:%j %U %W}", ymd), "061 08 09");
+  EXPECT_EQ(fmt::format("{:%G-W%V}", ymd), "2024-W09");
+
+  ymd = fmt::year_month_day(fmt::year(2021), fmt::month(1), fmt::day(1));
+  EXPECT_EQ(fmt::format("{:%a %j %U %W}", ymd), "Fri 001 00 00");
+  EXPECT_EQ(fmt::format("{:%G-W%V-%u}", ymd), "2020-W53-5");
+
+  ymd = fmt::year_month_day(fmt::year(1969), fmt::month(12), fmt::day(31));
+  EXPECT_EQ(fmt::format("{:%A %j}", ymd), "Wednesday 365");
+  EXPECT_EQ(fmt::format("{:%G-W%V}", ymd), "1970-W01");
+}