diff --git a/mercurial/changelog.py b/mercurial/changelog.py
--- a/mercurial/changelog.py
+++ b/mercurial/changelog.py
@@ -31,7 +31,10 @@ class changelog(revlog):
 
     def add(self, manifest, list, desc, transaction, p1=None, p2=None,
                   user=None, date=None):
-        if not date:
+        if date:
+            when, offset = map(int, date.split())
+            assert abs(offset) < 43200, 'bad time zone offset: %d' % offset
+        else:
             if time.daylight: offset = time.altzone
             else: offset = time.timezone
             date = "%d %d" % (time.time(), offset)