I don't know if it's still the case but once upon a time, the folks at DC Comics in charge of Superman said that his birthday was February 29. I'm not sure how they arrived at that date. I mean, I assume the editors there picked it because it was unique but I'm not sure how they figured Superman knew that. His home planet blew up, taking all birth records with it…and of course, there was no February on Krypton, nor did it probably rotate at a speed that required the insertion of an extra day after four years.
So maybe that was the date on Earth the day he was born on Krypton…except how would anyone know this? Superman could conceivably have figured out just when Krypton went kablooey but how could he have known how many days or weeks before that date he was born? So I'm guessing that he adopted as his birthdate, the day his rocketship landed on Earth. And I'm not sure DC Comics still holds to that date but just in case they do, many happy returns, Man of Steel. Use that super-breath to blow out the candles.