check start is not None before checking whether hour is before start

backfiller/backfiller/main.py

@@ -424,6 +424,7 @@ class BackfillerWorker(object):
 			for hour in hours:
 				# since backfilling can take a long time, recheck whether this
 				# hour is after the start
+				if self.start is not None:
 					if not isinstance(self.start, datetime.datetime):
 						start_hour = datetime.datetime.utcnow() - datetime.timedelta(hours=self.start)
 					else: