Well, if your manager can see your employee file (which they can.... certainly in Worlday) they will know your birthday. Oddly, at my employer, celebration of individual birthdays is discouraged, I have heard it is because it might make someone feel uncomfortable being called out for aging. As someone who is probably in the oldest 1% of the company... I find that amusingly curious. Birthdays are a good excuse for a team celebration, and that is almost always good team bonding.