diff -r 1c486516b39e Modules/_datetimemodule.c
--- a/Modules/_datetimemodule.c	Thu Dec 29 23:57:31 2016 -0700
+++ b/Modules/_datetimemodule.c	Fri Dec 30 06:05:47 2016 -0500
@@ -4251,6 +4251,7 @@
                            PyObject *tzinfo)
 {
     struct tm tm;
+    time_t fold_detection_time;
     int year, month, day, hour, minute, second, fold = 0;
 
     if (f(timet, &tm) != 0)
@@ -4274,13 +4275,32 @@
 
         result_seconds = utc_to_seconds(year, month, day,
                                         hour, minute, second);
-        /* Probe max_fold_seconds to detect a fold. */
-        probe_seconds = local(epoch + timet - max_fold_seconds);
+        fold_detection_time = epoch + timet - max_fold_seconds;
+#ifdef MS_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 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 truncate negative values
+        * on Windows. */
+        if (fold_detection_time - epoch < 0)
+            fold_detection_time = epoch;
+#endif
+        probe_seconds = local(fold_detection_time);
         if (probe_seconds == -1)
             return NULL;
         transition = result_seconds - probe_seconds - max_fold_seconds;
         if (transition < 0) {
-            probe_seconds = local(epoch + timet + transition);
+            fold_detection_time = epoch + timet + transition;
+#ifdef MS_WINDOWS
+            if (fold_detection_time - epoch < 0)
+                fold_detection_time = epoch + 1;
+#endif
+            probe_seconds = local(fold_detection_time);
             if (probe_seconds == -1)
                 return NULL;
             if (probe_seconds == result_seconds)