Do I qulify for 72 hr Visa?

from USA we have arrival in Beijing on 3/08/18 7:55 pm. When do my 72 hours start and finish? I need to know this because in a week or so I will be booking my ticket from Beijing to Manila, Kuala lumpur, Ho chi main city and back to Beijing to fly back to USA. (so far I only have a return flight from US to Beijing) Total 19 days trip. I really don't need to get out of the airport on the way back to Beijing but if I can for a day or so that will be nice. I want to keep my free entry for china so I can save some money. Thank you for all your help.