def get_published_date(self):
raise errors.Unimplemented()<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        Unpublished assets have no published date.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: (osid.calendaring.DateTime) - the published date
        raise:  IllegalState - ``is_published()`` is ``false``
        *compliance: mandatory -- This method must be implemented.*