@abc.abstractmethod<EOL><INDENT>def get_published_date(self):<DEDENT>
return<EOL>
Gets the published date of this asset.

        Unpublished assets have no published date. A published asset has
        a date available, however the date returned may indicate that
        not much is known.

        :return: the published date
        :rtype: ``osid.calendaring.DateTime``
        :raise: ``IllegalState`` -- ``is_published()`` is ``false``

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