diff --git a/flask_saml2/idp/sphandler.py b/flask_saml2/idp/sphandler.py
index fee4300..5e61d2a 100644
--- a/flask_saml2/idp/sphandler.py
+++ b/flask_saml2/idp/sphandler.py
@@ -290,7 +290,7 @@ def format_datetime(self, value: datetime.datetime) -> str:
         formatting, and don't support the format produced by
         :meth:`datetime.datetime.isoformat`.
         """
-        return value.isoformat()
+        return value.replace(tzinfo=None).isoformat(timespec='seconds')
 
     def __str__(self):
         if self.display_name:
diff --git a/flask_saml2/sp/idphandler.py b/flask_saml2/sp/idphandler.py
index 50788ad..c93d2c8 100644
--- a/flask_saml2/sp/idphandler.py
+++ b/flask_saml2/sp/idphandler.py
@@ -266,7 +266,7 @@ def format_datetime(self, value: datetime.datetime) -> str:
         formatting, and don't support the format produced by
         :meth:`datetime.datetime.isoformat`.
         """
-        return value.isoformat()
+        return value.replace(tzinfo=None).isoformat(timespec='seconds') + 'Z'
 
     def __str__(self):
         if self.display_name: