diff --git a/oslo_versionedobjects/_utils.py b/oslo_versionedobjects/_utils.py
index 41eec0c3..fed228b8 100644
--- a/oslo_versionedobjects/_utils.py
+++ b/oslo_versionedobjects/_utils.py
@@ -25,5 +25,6 @@ def isotime(at):
     """Stringify time in ISO 8601 format."""
     st = at.strftime(_ISO8601_TIME_FORMAT)
     tz = at.tzinfo.tzname(None) if at.tzinfo else 'UTC'
-    st += ('Z' if tz == 'UTC' else tz)
+    # Need to handle either iso8601 or python UTC format
+    st += ('Z' if tz in ['UTC', 'UTC+00:00'] else tz)
     return st