upstream change:
https://www.mercurial-scm.org/repo/hg/rev/c6061cadb400

minimal fix:

diff --git a/jcheck.py b/jcheck.py
--- a/jcheck.py
+++ b/jcheck.py
@@ -45,7 +45,7 @@
 
 import sys, os, re, urllib, urllib2
 from mercurial.node import *
-from mercurial import cmdutil, patch, util, context, templater
+from mercurial import cmdutil, patch, util, context, templater, utils
 try:
     # Mercurial 4.3 and higher
     from mercurial import registrar
@@ -58,7 +58,7 @@ Fail = True
 
 def datestr(ctx):
     # Mercurial 0.9.5 and earlier append a time zone; strip it.
- return util.datestr(ctx.date(), format="%Y-%m-%d %H:%M")[:16]
+ return utils.dateutil.datestr(ctx.date(), format="%Y-%m-%d %H:%M")[:16]
 
 def oneline(ctx):
     return ("%5d:%s %-12s %s %s\n"