diff --git a/keystoneclient/utils.py b/keystoneclient/utils.py
index 67609e14c..d71974b55 100644
--- a/keystoneclient/utils.py
+++ b/keystoneclient/utils.py
@@ -114,7 +114,7 @@ def isotime(at=None, subsecond=False):
                      if not subsecond
                      else _ISO8601_TIME_FORMAT_SUBSECOND)
     tz = at.tzinfo.tzname(None) if at.tzinfo else 'UTC'
-    st += ('Z' if tz == 'UTC' else tz)
+    st += ('Z' if (tz == 'UTC' or tz == 'UTC+00:00') else tz)
     return st