None of us really knows his or her birthday--we just know what we were told by our parents, that such-and-such a day was the day we were born.And then as an adult, you can order a copy of your own birth certificate, and see that what you were told and what is on the document indeed match.
Right?
I thought I knew my grandfather's birthday--then, after his death, discovered that the baptismal record has a date two days earlier than that. I think the baptismal record must be in error but had no one to ask--anyone else who might have known was long dead. I did find that his WWI registration gave the date of birth that I had always assumed was his correct birthday.
I suppose if a person is born in a hospital the staff supplies the information for the birth certificate, but in earlier times when most babies were born at home, it would have been up to the parents to report the date of birth either for a baptismal record or for a civil registration (a more recent practice).