Retrieve original value and check it wasn't signed more
        than max_age seconds ago.

def unsign(self, value, max_age=None):
        """
        Retrieve original value and check it wasn't signed more
        than max_age seconds ago.
        """
        result = super(TimestampSigner, self).unsign(value)
        value, timestamp = result.rsplit(self.sep, 1)
        timestamp = baseconv.base62.decode(timestamp)
        if max_age is not None:
            if isinstance(max_age, datetime.timedelta):
                max_age = max_age.total_seconds()
            # Check timestamp is not older than max_age
            age = time.time() - timestamp
            if age > max_age:
                raise SignatureExpired(
                    'Signature age %s > %s seconds' % (age, max_age))
        return value