Calculating time and area for qulifications of 72 hour visa free

I will be arriving from Bangkok to Beijing at 7:05 am on Mar 18. I will be departing Beijing for Seattle on Tues Mar 21 at 3:50 PM. This is actually longer than 72 hours. Do I have until 11:59 PM on Mar 21 to depart and qualify? I want to go to downtown and the Great Wall will I be ok to travel to those destinations? Or is the 72 hour free transit for staying at the airport only