[Leonhardt sadly knows that one all too well. He was a mere child when he lost his whole family and didn't recall when his birthday was because he was too young to know it himself and the circumstances of it all didn't aid his memory.]

A birthday is merely a date that serves as a way to count the years you have lived to celebrate them. If you cannot recall what that should be, it doesn't matter since the date only aids in counting. The only important thing is that you can recall the day you picked.