'Return the creation time of a valid Fernet token.
:type fernet_token: six.text_type'

@classmethod
def creation_time(cls, fernet_token):

    fernet_token = TokenFormatter.restore_padding(fernet_token)
    token_bytes = base64.urlsafe_b64decode(fernet_token.encode('utf-8'))
    timestamp_bytes = token_bytes[TIMESTAMP_START:TIMESTAMP_END]
    timestamp_int = struct.unpack('>Q', timestamp_bytes)[0]
    issued_at = datetime.datetime.utcfromtimestamp(timestamp_int)
    return issued_at
