def get_expiration(self):
if not self.has_expiration():<EOL><INDENT>raise IllegalState()<EOL><DEDENT>else:<EOL><INDENT>raise Unimplemented()<EOL><DEDENT>
Gets the expiration date associated with this authentication credential.

        Consumers should check for the existence of a an expiration
        mechanism via ``hasExpiration()``.

        :return: the expiration date of this authentication credential
        :rtype: ``timestamp``
        :raise: ``IllegalState`` -- ``has_expiration()`` is ``false``

        *compliance: mandatory -- This method must be implemented.*