“Buddha Sāsana” has a different meaning from Buddha Dhamma.
– “Buddha Sāsana” refers to the geographic locations/time periods where the true teachings of the Buddha (Buddha Dhamma) are present.
– For example, the “Buddha Sāsana” of the previous Buddha (Buddha Kassapa) disappeared long before the current “Buddha Sāsana” of Buddha Gotama was established with him attaining Enlightenment. Of course, all Buddhas teach the same Buddha Dhamma/Paticca Samuppada/Tilakkhana.
– The current “Buddha Sāsana” of Buddha Gotama is supposed to last only about 2500 more years. After that, his true teachings will fade away. As we know, even now, true teachings are not widely available.