Merge branch 'release_21.05' of https://github.com/galaxyproject/galaxy into alex_branch

lib/galaxy/webapps/galaxy/api/history_contents.py

@@ -1,11 +1,13 @@
 """
 API operations on the contents of a history.
 """
+import datetime
 import json
 import logging
 import os
 import re
-from datetime import datetime
+
+import dateutil.parser
 
 from galaxy import (
     exceptions,
@@ -1082,8 +1084,10 @@ class HistoryContentsController(BaseGalaxyAPIController, UsesLibraryMixinItems,
         # if it hasn't then we can short-circuit the poll request
         since = kwd.get('update_time-gt', None)
         if since:
-            since_str = self.history_contents_filters.parse_date(since)
-            since_date = datetime.fromisoformat(since_str)
+            # sqlalchemy DateTime columns are not timezone aware, so parse `since` into timezone-aware
+            # datetime and then convert to naive datetime object representing UTC,
+            # assuming history.update_time represents UTC time.
+            since_date = dateutil.parser.isoparse(since).astimezone(datetime.timezone.utc).replace(tzinfo=None)
             if history.update_time <= since_date:
                 trans.response.status = 204
                 return