diff -r 4de15c54e59f -r c6061cadb400 mercurial/subrepo.py
--- a/mercurial/subrepo.py	Thu Feb 08 23:27:24 2018 +0530
+++ b/mercurial/subrepo.py	Thu Feb 15 17:18:26 2018 +0100
@@ -36,6 +36,7 @@
     util,
     vfs as vfsmod,
 )
+from .utils import dateutil
 
 hg = None
 reporelpath = subrepoutil.reporelpath
@@ -1467,7 +1468,7 @@
         if date:
             # git's date parser silently ignores when seconds < 1e9
             # convert to ISO8601
-            env['GIT_AUTHOR_DATE'] = util.datestr(date,
+            env['GIT_AUTHOR_DATE'] = dateutil.datestr(date,
                                                   '%Y-%m-%dT%H:%M:%S %1%2')
         self._gitcommand(cmd, env=env)
         # make sure commit works otherwise HEAD might not exist under certain