def getOldestRequestTime(self):
        """Returns the submitted_at of the oldest unclaimed build request for
        this builder, or None if there are no build requests.

        @returns: datetime instance or None, via Deferred
        """
        bldrid = yield self.getBuilderId()
        unclaimed = yield self.master.data.get(
            ('builders', bldrid, 'buildrequests'),
            [resultspec.Filter('claimed', 'eq', [False])],
            order=['submitted_at'], limit=1)
        if unclaimed:
            return unclaimed[0]['submitted_at']