Forego fold detection for timestamps in the range [0, 86399] on Windows.

On Windows, passing a negative value to local results in an OSError because localtime_s on Windows does not support negative timestamps. Unfortunately this means that fold detection for timestamps between 0 and max_fold_seconds will result in this OSError since we subtract max_fold_seconds from the timestamp to detect a fold. However, since we know there haven't been any folds in the interval [0, max_fold_seconds) in any timezone, we can hackily just forego fold detection for this time range on Windows.

Author: ammaraskar

Modules/_datetimemodule.c

@@ -4625,7 +4625,21 @@ datetime_from_timet_and_us(PyObject *cls, TM_FUNC f, time_t timet, int us,
     second = Py_MIN(59, tm.tm_sec);
 
     /* local timezone requires to compute fold */
-    if (tzinfo == Py_None && f == _PyTime_localtime) {
+    if (tzinfo == Py_None && f == _PyTime_localtime
+    /* On Windows, passing a negative value to local results
+    * in an OSError because localtime_s on Windows does
+    * not support negative timestamps. Unfortunately this
+    * means that fold detection for time values between
+    * 0 and max_fold_seconds will result in an identical
+    * error since we subtract max_fold_seconds to detect a
+    * fold. However, since we know there haven't been any
+    * folds in the interval [0, max_fold_seconds) in any
+    * timezone, we can hackily just forego fold detection
+    * for this time range on Windows. */
+#ifdef MS_WINDOWS
+        && (timet - max_fold_seconds > 0)
+#endif
+    	) {
         long long probe_seconds, result_seconds, transition;
 
         result_seconds = utc_to_seconds(year, month, day,