diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -440,7 +440,14 @@ def versiontuple(v=None, n=4):
 
 # used by parsedate
 defaultdateformats = (
-    '%Y-%m-%d %H:%M:%S',
+    '%Y-%m-%dT%H:%M:%S', # the 'real' ISO8601
+    '%Y-%m-%dT%H:%M',    #   without seconds
+    '%Y-%m-%dT%H%M%S',   # another awful but legal variant without :
+    '%Y-%m-%dT%H%M',     #   without seconds
+    '%Y-%m-%d %H:%M:%S', # our common legal variant
+    '%Y-%m-%d %H:%M',    #   without seconds
+    '%Y-%m-%d %H%M%S',   # without :
+    '%Y-%m-%d %H%M',     #   without seconds
     '%Y-%m-%d %I:%M:%S%p',
     '%Y-%m-%d %H:%M',
     '%Y-%m-%d %I:%M%p',
diff --git a/tests/test-parse-date.t b/tests/test-parse-date.t
--- a/tests/test-parse-date.t
+++ b/tests/test-parse-date.t
@@ -258,3 +258,31 @@ Test issue 3764 (interpreting 'today' an
   $ hg log -d today --template '{desc}\n'
   Explicitly committed now.
   today is a good day to code
+
+Test parsing various ISO8601 forms
+
+  $ hg debugdate "2016-07-27T12:10:21"
+  internal: 1469646621 * (glob)
+  standard: Wed Jul 27 12:10:21 2016 -0700
+  $ hg debugdate "2016-07-27T12:10:21Z"
+  internal: 1469621421 0
+  standard: Wed Jul 27 12:10:21 2016 +0000
+  $ hg debugdate "2016-07-27T12:10:21+00:00"
+  internal: 1469621421 0
+  standard: Wed Jul 27 12:10:21 2016 +0000
+  $ hg debugdate "2016-07-27T121021Z"
+  internal: 1469621421 0
+  standard: Wed Jul 27 12:10:21 2016 +0000
+
+  $ hg debugdate "2016-07-27 12:10:21"
+  internal: 1469646621 * (glob)
+  standard: Wed Jul 27 12:10:21 2016 -0700
+  $ hg debugdate "2016-07-27 12:10:21Z"
+  internal: 1469621421 0
+  standard: Wed Jul 27 12:10:21 2016 +0000
+  $ hg debugdate "2016-07-27 12:10:21+00:00"
+  internal: 1469621421 0
+  standard: Wed Jul 27 12:10:21 2016 +0000
+  $ hg debugdate "2016-07-27 121021Z"
+  internal: 1469621421 0
+  standard: Wed Jul 27 12:10:21 2016 +0000