Just got one of my Thrifty hits:
10/29/2011 11/08/11 THRIFTY CAR RENTAL BONUS 50
Looks like their cutoff is Saturday, with the miles posting on the 2nd Tuesday following. I had another on the 30th that hasn't posted yet. I returned the car the same day, so the date could be either one. So far, though, all the car companies have used the pick-up date as the activity date.