def getNewestCompleteTime(self):
        """Returns the complete_at of the latest completed build request for
        this builder, or None if there are no such build requests.

        @returns: datetime instance or None, via Deferred
        """
        bldrid = yield self.getBuilderId()
        completed = yield self.master.data.get(
            ('builders', bldrid, 'buildrequests'),
            [resultspec.Filter('complete', 'eq', [False])],
            order=['-complete_at'], limit=1)
        if completed:
            return completed[0]['complete_at']
        else:
            return None
Returns the complete_at of the latest completed build request for
        this builder, or None if there are no such build requests.

        @returns: datetime instance or None, via Deferred