I believe
All FPs are done electronically, so the day you do you FP it goes to FBI database and in most cases it comes back to INS on the same day. At least it happen to me two times, FP done -> goes to FBI on same day -> FBI sent them back to INS on same day.
So consider your FP date is done deal and count 15 months from that date only. You wont be able to argue INS over this.
Forget HR, HR is not INS. I am 100% sure about 15 month expiration.