gh-148658: Fix DST detection for negative timestamps on Windows

Author: Jiekang Tian

Lib/test/datetimetester.py

@@ -5663,12 +5663,34 @@ def test_astimezone_default_near_fold(self):
 
     @unittest.skipIf(sys.platform != "win32", "gh-148658 only affects Windows")
     def test_astimezone_negative_timestamp_dst(self):
-        # gh-148658: astimezone() returned DST name for negative timestamps
-        # on Windows. Verify that the timezone name is consistent between
-        # negative and non-negative timestamps.
-        dt_neg1 = self.theclass.fromtimestamp(-1).astimezone()
-        dt_zero = self.theclass.fromtimestamp(0).astimezone()
-        self.assertEqual(dt_neg1.tzname(), dt_zero.tzname())
+        # gh-148658: astimezone() returned incorrect DST name for negative
+        # timestamps on Windows. Verify C layer returns correct tm_zone
+        # by checking astimezone() produces correct timezone names.
+        import time
+        winter_ts = -1  # 1969-12-31
+        summer_ts = -15897600  # 1969-07-01
+
+        # Get expected timezone names from time.localtime
+        winter_local = time.localtime(winter_ts)
+        summer_local = time.localtime(summer_ts)
+
+        dt_winter = self.theclass.fromtimestamp(winter_ts).astimezone()
+        dt_summer = self.theclass.fromtimestamp(summer_ts).astimezone()
+
+        # Verify astimezone() returns correct timezone names
+        self.assertEqual(dt_winter.tzname(), winter_local.tm_zone)
+        self.assertEqual(dt_summer.tzname(), summer_local.tm_zone)
+
+        # For DST timezones, winter and summer should have different names
+        # For non-DST timezones, names should be the same
+        if winter_local.tm_zone == summer_local.tm_zone:
+            # Non-DST timezone: both should be standard time
+            self.assertEqual(winter_local.tm_isdst, 0)
+            self.assertEqual(summer_local.tm_isdst, 0)
+        else:
+            # DST timezone: one should be standard, one DST
+            self.assertEqual(winter_local.tm_isdst, 0)
+            self.assertEqual(summer_local.tm_isdst, 1)
 
     def test_aware_subtract(self):
         cls = self.theclass

Python/pytime.c

@@ -357,18 +357,37 @@ _PyTime_windows_filetime(time_t timer, struct tm *tm, int is_local)
     tm->tm_yday = _PyTime_calc_yday(&st_result);
 
     /* DST flag: -1 (unknown) for local time on historical dates, 0 for UTC.
-     * For timezones that don't observe DST, set tm_isdst to 0 to ensure
-     * strftime('%Z') returns the standard time name (gh-148658). */
+     * Detect DST by comparing standard time with local time (gh-148658). */
     if (is_local) {
         TIME_ZONE_INFORMATION tzi;
         DWORD tz_result = GetTimeZoneInformation(&tzi);
-        if (tz_result != TIME_ZONE_ID_INVALID &&
+        if (tz_result == TIME_ZONE_ID_INVALID ||
             tzi.DaylightDate.wMonth == 0) {
             /* Timezone does not observe DST */
             tm->tm_isdst = 0;
         }
         else {
-            tm->tm_isdst = -1;
+            /* Calculate standard time from UTC and compare with local time. */
+            ULONGLONG utc_ticks = ((ULONGLONG)timer + SECS_BETWEEN_EPOCHS) *
+                                   HUNDRED_NS_PER_SEC;
+            LONGLONG standard_offset_ticks = (LONGLONG)tzi.Bias * 60LL *
+                                              HUNDRED_NS_PER_SEC;
+            ULONGLONG standard_ticks = utc_ticks - standard_offset_ticks;
+            FILETIME ft_standard;
+            ft_standard.dwLowDateTime = (DWORD)(standard_ticks);
+            ft_standard.dwHighDateTime = (DWORD)(standard_ticks >> 32);
+            SYSTEMTIME st_standard;
+            if (!FileTimeToSystemTime(&ft_standard, &st_standard)) {
+                PyErr_SetFromWindowsErr(0);
+                return -1;
+            }
+            if (st_result.wHour == st_standard.wHour &&
+                st_result.wMinute == st_standard.wMinute) {
+                tm->tm_isdst = 0;
+            }
+            else {
+                tm->tm_isdst = 1;
+            }
         }
     }
     else {