My trip to Thailand from USA has a stop in Guangzhou. Can I stay for 72hrs on the way back home?

Hello,
My family is planning a trip to Thailand and we noticed that our flight has a stop in Guangzhou, China on the way there and back to the USA. On our way back home, we would love to visit Guangzhou. May we stay for 72 hours in Guangzhou visa free from April 15 - April 18? Our flight leaves Bangkok, and arrives in Guangzhou at 7:30 p.m. and then departs to the USA at 9:30 p.m that same day. We would like to change our flight to stay for 72 hours in Guangzhou visa free, then fly home. Do we qualify for the 72 hour visa free transit if we depart Guangzhou at 9:30p.m. on the 3rd day? The flight departs 2 hours beyond the 72 hours, is that ok? Or would we need to depart a day earlier to avoid going over 2 hours?