Duration of stay in Beijing

I would like to take advantage of the 72 hours free transit to Beijing, the schedule flight I have is Leaving NY on the 16th of Oct, arriving being on the 17th at 6:20 PM, I really want to stay there for the 3 day tour, but to be able to do that, I will exceed the limit, my schedule flight is on the 20th of Oct. at 8Pm., that would total to about 74 Hours. wolud I be able to have this extra 2 hours?????