<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def as_of(self, qtime=None):
 """ 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 """

        clone = self._clone()
        clone.querytime = QueryTime(time=qtime, active=True)
        return clone