Originally Posted by Spotted
I think that has to do with TB's for racing purposes. They want the foals born closest to January, so they are bigger for race day as a 2yr old.
However your filly is still a yearling this year and still a 2 year old in 2013.
It's not just for TBs. Any breed with a futurity is figured that way. As LadyDreamer said, that is the reason why show farms breed specifically to have a foal in the early part of the year.