Longer than 72 hours but seems to be with in the rules

My flight arrives PEK at 7:05 am on Mar 18. My continuing flight departs PEK for SEA on Mar 21 at 15:50.
Does this qualify for the 72 hour free transit? If the 72 hours starts from 00:00 the day following the entry date that would be Mar 19, then I have until 23:59 on Mar 21 to depart. correct? you say to play it safe. this is beyond the "strict 72 hour" limit. Do most people stay within the strict limit? What do you advise? The time is close. Should I pay to get visas? I hate to have to pay for them if it is not necessary.