What was the Georgian calendar date when the moon was split?

I'm reading this: > People living in other parts of the world witnessed the splitting of > the moon as well, which happened 18 June 1178, corresponding with > Dhul-Hijjah 29, 573 on the Islamic calendar. https://islamicreminder.org/the-splitting-of-the-moon-witnessed-in-other-parts-of-the-world/ I believe this **Dhul-Hijjah 29, 573** is a reference to an Islamic date. However, I think Islamic calendar is based on the day Prophet emigrated. So how can it be Islamic year 573 if miracle happened in his lifetime, Am I missing something?