Test ambiguous time

Author: vitaut

.github/workflows/linux.yml

@@ -58,7 +58,7 @@ jobs:
     - uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938 # v4.2.0
 
     - name: Set timezone
-      run: sudo timedatectl set-timezone 'Europe/Minsk'
+      run: sudo timedatectl set-timezone 'Europe/Kyiv'
 
     - name: Install GCC 4.9
       run: |

include/fmt/chrono.h

@@ -2320,15 +2320,20 @@ struct formatter<local_time<Duration>, Char> : formatter<std::tm, Char> {
   template <typename FormatContext>
   auto format(local_time<Duration> val, FormatContext& ctx) const
       -> decltype(ctx.out()) {
+    auto time_since_epoch = val.time_since_epoch();
+    auto seconds_since_epoch =
+        detail::duration_cast<std::chrono::seconds>(time_since_epoch);
+    // Use gmtime to prevent time conversion since local_time has an
+    // unspecified time zone.
+    auto t = gmtime(seconds_since_epoch.count());
     using period = typename Duration::period;
     if (period::num == 1 && period::den == 1 &&
         !std::is_floating_point<typename Duration::rep>::value) {
-      return formatter<std::tm, Char>::format(localtime(val), ctx);
+      return formatter<std::tm, Char>::format(t, ctx);
     }
-    auto epoch = val.time_since_epoch();
-    auto subsecs = detail::duration_cast<Duration>(
-        epoch - detail::duration_cast<std::chrono::seconds>(epoch));
-    return formatter<std::tm, Char>::do_format(localtime(val), ctx, &subsecs);
+    auto subsecs =
+        detail::duration_cast<Duration>(time_since_epoch - seconds_since_epoch);
+    return formatter<std::tm, Char>::do_format(t, ctx, &subsecs);
   }
 };
 

test/chrono-test.cc

@@ -428,6 +428,18 @@ TEST(chrono_test, local_system_clock_time_point) {
 
 #endif  // FMT_USE_LOCAL_TIME
 
+TEST(chrono_test, daylight_savings_time_end) {
+  // 2024-10-27 03:05 as the number of seconds since epoch in Europe/Kyiv time.
+  // It is slightly after the DST end and passing it to to_sys will result in
+  // an ambiguous time error:
+  //   2024-10-27 03:05:00 is ambiguous.  It could be
+  //   2024-10-27 03:05:00 EEST == 2024-10-27 00:05:00 UTC or
+  //   2024-10-27 03:05:00 EET == 2024-10-27 01:05:00 UTC
+  auto t =
+      fmt::local_time<std::chrono::seconds>(std::chrono::seconds(1729998300));
+  EXPECT_EQ(fmt::format("{}", t), "2024-10-27 03:05:00");
+}
+
 #ifndef FMT_STATIC_THOUSANDS_SEPARATOR
 
 TEST(chrono_test, format_default) {
@@ -1033,7 +1045,7 @@ TEST(chrono_test, glibc_extensions) {
   {
     auto t = std::tm();
     t.tm_year = -5 - 1900;
-    EXPECT_EQ(fmt::format( "{:%Y}", t), "-005");
+    EXPECT_EQ(fmt::format("{:%Y}", t), "-005");
     EXPECT_EQ(fmt::format("{:%_Y}", t), "  -5");
     EXPECT_EQ(fmt::format("{:%-Y}", t), "-5");
   }
@@ -1045,8 +1057,6 @@ TEST(chrono_test, glibc_extensions) {
     EXPECT_EQ(fmt::format("{:%_m}", t), " 7");
     EXPECT_EQ(fmt::format("{:%-m}", t), "7");
   }
-
-
 }
 
 TEST(chrono_test, out_of_range) {