def expires_at(self):
        """
        A :py:obj:`~datetime.datetime` of when this signature expires, if a signature expiration date is specified.
        Otherwise, ``None``
        """
        if 'SignatureExpirationTime' in self._signature.subpackets:
            expd = next(iter(self._signature.subpackets['SignatureExpirationTime'])).expires
            return self.created + expd
        return None