code: def as_of(self, qtime=None):
          
          clone = self._clone()
          clone.querytime = QueryTime(time=qtime, active=True)
          return clone
docs: Sets the time for which we want to retrieve an object.
  
          :param qtime: The UTC date and time; if None then use the current
              state (where version_end_date = NULL)
          :return: A VersionedQuerySet