It should be the third, fourth and fifth digits of your serial number. Mine is 921 which would mean the 9 is for 2009 and the 21 is the 21st week of the year.
912. So it was made sometime in March '09? That doesn't seem right…
edit: I forget this is a replacement phone, my original was a release day pick-up, hence thinking March '08, and it couldn't have been made that early!