def _get_date_end(self, pricelist_id):
        pool = pooler.get_pool(self.cr.dbname)
        pricelist = pool.get('product.pricelist.version').read(self.cr, self.uid, [pricelist_id], ['date_end'], context=self.localcontext)[0]
        return pricelist['date_end']  

<para style="8_left">[[ formatLang (get_date_end(data['form']['price_list']), date=True) ]]</para>

That works at simple price lists, but not if the price list based on another price list.