diff -r 6a7ff5816c5f -r 31750413f8d7 mercurial/formatter.py
--- a/mercurial/formatter.py	Tue Apr 17 03:07:01 2018 +0200
+++ b/mercurial/formatter.py	Fri Apr 13 22:58:49 2018 +0900
@@ -144,7 +144,9 @@
     @staticmethod
     def formatdate(date, fmt):
         '''convert date tuple to appropriate format'''
-        return date
+        # timestamp can be float, but the canonical form should be int
+        ts, tz = date
+        return (int(ts), tz)
     @staticmethod
     def formatdict(data, key, value, fmt, sep):
         '''convert dict or key-value pairs to appropriate dict format'''