diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -1023,6 +1023,9 @@ def datestr(date=None, format='%a %b %d 
     number of seconds away from UTC. if timezone is false, do not
     append time zone to string."""
     t, tz = date or makedate()
+    if t < 0:
+        t = 0   # time.gmtime(lt) fails on Windows for lt < -43200
+        tz = 0
     if "%1" in format or "%2" in format:
         sign = (tz > 0) and "-" or "+"
         minutes = abs(tz) // 60