I was wondering the same thing, 3 days vs. 72 hours. With our projected landing time around 3:30 this Thursday, we wondered if we needed to wait until 3:30 Monday to submit the form. We went ahead and submitted it yesterday morning at 9:00, which is outside 72 hours but still within 3 days, figuring if it was too early they would notify us. Not a problem - my husband's approval came back around 10 am, and mine came through at 2 pm. So it looks like 3 calendar days should be good to go.