diff --git a/numbas_lti/models.py b/numbas_lti/models.py
index bcd6e71..e636ff3 100644
--- a/numbas_lti/models.py
+++ b/numbas_lti/models.py
@@ -42,7 +42,7 @@
 from .groups import group_for_attempt, group_for_resource_stats, group_for_resource
 from .report_outcome import report_outcome, report_outcome_for_attempt, ReportOutcomeException
 from .diff import make_diff, apply_diff
-from .util import parse_scorm_timeinterval, iso_time
+from .util import parse_scorm_timeinterval, iso_time, time_from_iso
 from .examparser import numbasobject
 
 requests = requests_session.get_session()
@@ -848,14 +848,9 @@ def get_lti_13_lineitem(self, create = False):
 
         saved_lineitem = LineItem(saved_lineitem)
 
-        def parse_isoformat(time: str):
-            if time is None:
-                return None
-            return datetime.fromisoformat(time)
-
         if (saved_lineitem.get_score_maximum() != lineitem.get_score_maximum()
-            or parse_isoformat(saved_lineitem.get_start_date_time()) != parse_isoformat(lineitem.get_start_date_time())
-            or parse_isoformat(saved_lineitem.get_end_date_time()) != parse_isoformat(lineitem.get_end_date_time())
+            or time_from_iso(saved_lineitem.get_start_date_time()) != time_from_iso(lineitem.get_start_date_time())
+            or time_from_iso(saved_lineitem.get_end_date_time()) != time_from_iso(lineitem.get_end_date_time())
             or saved_lineitem.get_tag() != lineitem.get_tag()
             ):
             max_score = lineitem.get_score_maximum()
diff --git a/numbas_lti/util.py b/numbas_lti/util.py
index 6cdb894..7a48997 100644
--- a/numbas_lti/util.py
+++ b/numbas_lti/util.py
@@ -97,3 +97,9 @@ def iso_time(time: datetime) -> str:
         Convert a datetime to an ISO format string if it's not None, otherwise return None.
     """
     return time.isoformat() if time else None
+
+def time_from_iso(time: str) -> datetime:
+    """
+        Parse an ISO format datetime string if the argument is not None, otherwise return None.
+    """
+    return datetime.fromisoformat(time) if time is not None else None