It's similar to horse racing, where every horse's age is based on the year of its birth, not the actual date.
I recall some old TV show in which a thoroughbred was about to foal on New Year's Eve, and the owners rushed her in a trailer over the time zone so the horse would be born in the next year. I seem to recall Dennis Weaver as one of the actors (so there's a track connection) Jim Gerweck Running Times
