TMSS-60: start/stop time are python datetimes, not strings

SAS/TMSS/test/tmss_test_data_django_models.py

@@ -258,10 +258,10 @@ def Subtask_test_data(task_blueprint: models.TaskBlueprint=None, subtask_templat
         specifications_doc = get_default_json_object_for_schema(subtask_template.schema)
 
     if start_time is None:
-        start_time = datetime.utcnow().isoformat()
+        start_time = datetime.utcnow()
 
     if stop_time is None:
-        stop_time = datetime.utcnow().isoformat()
+        stop_time = datetime.utcnow()
 
     if cluster is None:
         cluster = models.Cluster.objects.create(name="dummy cluster", location="downstairs", tags=[])