code: def _get_oldest_event_timestamp(self):
          
          # Retrieve the oldest event in order to start aggregation
          # from there
          query_events = Search(
              using=self.client,
              index=self.event_index
          )[0:1].sort(
              {'timestamp': {'order': 'asc'}}
          )
          result = query_events.execute()
          # There might not be any events yet if the first event have been
          # indexed but the indices have not been refreshed yet.
          if len(result) == 0:
              return None
          return parser.parse(result[0]['timestamp'])
docs: Search for the oldest event timestamp.