Where can I find more information about the 72 hour visa free policy?

We are Canadian citizens. We plan to drop by Beijing on our way back to Canada. The plane arrives in Beijing on June 3 at 11:00am and departs for Toronto on June 6 at 1:00pm. Based on this flight schedule, we are 2 hours past the 72hour policy. Is the 72h based on the flight itinerary or when the documents are stamped? Please advise. Thank you!