@abc.abstractmethod<EOL><INDENT>def get_time_period_id(self):<DEDENT>
return<EOL>
Gets the ``TimePeriod Id`` for this recurring event.

        A ``Schedule`` with an associated ``TimePeriod`` overrides any
        start or end date set.

        :return: the time period ``Id``
        :rtype: ``osid.id.Id``
        :raise: ``IllegalState`` -- ``has_time_period()`` is ``false``

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