Regarding this 72 hrs visa waiver program, how they calculate this?

Generally speaking, the 72-hour starts from the scheduled landing time and ends on the scheduled departure time. But when the policy is carried out, different airports have a little bit difference.
For your arrival and departure time, the total staying time is over 72 hours, you may need to apply for a Transit G for your transfering. To make it sure, you are still suggested to consult the airport directly for real situation.