I was flipping through some wikis and saw that the Final Fantasy VII characters have birthdays with their year stated. After browsing around for a little bit, I couldn't figure out where they got the year date from. I was hoping anyone here can point me in the right direction?

I.E.
Yuffie Kisaragi's birth date is November 20, 1991.
Barret Wallace's birth date is December 15, 1972