72 hour transit stopover bejing

Hello. We will fligh from ho chi Ming city to Vancouver via Beijing. Our stopover from the arrival time to the departure time is exactly 74 hours and 20 minutes.
I read that the 72 hours are calculated from 12 am of the next day so we would be within the 72 hours is that right? If not is it just possible to wait for 2.20 hours at the airport and apply than for a visa to be within the 72 hours?
Thanks for your help