def _has_expired(self):
        """ Has this HIT expired yet? """
        expired = False
        if hasattr(self, 'Expiration'):
            now = datetime.datetime.utcnow()
            expiration = datetime.datetime.strptime(self.Expiration, '%Y-%m-%dT%H:%M:%SZ')
            expired = (now >= expiration)
        else:
            raise ValueError("ERROR: Request for expired property, but no Expiration in HIT!")
        return expired
Has this HIT expired yet?