diff --git a/src/saml2/time_util.py b/src/saml2/time_util.py
index 81987ef..e0cc5d3 100644
--- a/src/saml2/time_util.py
+++ b/src/saml2/time_util.py
@@ -16,7 +16,7 @@ from datetime import datetime
 
 TIME_FORMAT = "%Y-%m-%dT%H:%M:%SZ"
 TIME_FORMAT_WITH_FRAGMENT = re.compile(
-    "^(\d{4,4}-\d{2,2}-\d{2,2}T\d{2,2}:\d{2,2}:\d{2,2})\.\d*Z$")
+    "^(\d{4,4}-\d{2,2}-\d{2,2}T\d{2,2}:\d{2,2}:\d{2,2})(\.\d)*Z?$")
 
 # ---------------------------------------------------------------------------
 # I'm sure this is implemented somewhere else can't find it now though, so I