72 hr free visa beijing

I am arriving in Beijing 11th sept at 1130 pm flight from the USA. I am taking advantage of the 72 hr layover
since my 3rd country is Manila , Philippines. However my question is : My flight to Manila is 0030 on 15th sept ( this is actually 73 hrs) to be exact. I should already be at the airport by 9 PM on the 14th sept for Manila trip which technically is less than 72 hrs. outside of Beijing airport. I have read somewhere that the 72 hrs starts from the time they stamped the passport on arrival not time of plane landing. I could hang around at the passport control area for 1 hr if necessary.Any thoughts? Thanks