code: def _calculate_bounds(self):
          
          # get start datetime
          first = self.coll_handle.find_one(None, sort=[("ts", ASCENDING)])
          last = self.coll_handle.find_one(None, sort=[("ts", DESCENDING)])
  
          self._start = first['ts']
          if self._start.tzinfo is None:
              self._start = self._start.replace(tzinfo=tzutc())
  
          self._end = last['ts']
          if self._end.tzinfo is None:
              self._end = self._end.replace(tzinfo=tzutc())
  
          return True
docs: Calculate beginning and end of log events.