code: def get_bookmark(self):
          
          if not Index(self.aggregation_alias,
                       using=self.client).exists():
              if not Index(self.event_index,
                           using=self.client).exists():
                  return datetime.date.today()
              return self._get_oldest_event_timestamp()
  
          # retrieve the oldest bookmark
          query_bookmark = Search(
              using=self.client,
              index=self.aggregation_alias,
              doc_type=self.bookmark_doc_type
          )[0:1].sort(
              {'date': {'order': 'desc'}}
          )
          bookmarks = query_bookmark.execute()
          # if no bookmark is found but the index exist, the bookmark was somehow
          # lost or never written, so restart from the beginning
          if len(bookmarks) == 0:
              return self._get_oldest_event_timestamp()
  
          # change it to doc_id_suffix
          bookmark = datetime.datetime.strptime(bookmarks[0].date,
                                                self.doc_id_suffix)
          return bookmark
docs: Get last aggregation date.