Beijing - how is the 72 hour period calculated?

I am reading contradictory information about when the calculation of the 72 hour period starts for transit in Beijing. Some sites are saying the calculation is based on scheduled arrival and departure times so a strict 72 hours between arrival & departure, but I just read on Trip Advisor that the calculation starts at 00h01 following the day of entry. So, three full days starting the next day for a potential stay greater than 72 hours, please could you confirm which is correct? This will greatly affect the flights that I book!

''As of April 30, 2016 this "starts at midnight after arrival" is confirmed for Beijing. As long as your flight out is any time on the day that 72 hours expires, you will have no problems at immigration.'' but have not seen this on any 'official' sites, please could you confirm?

Thank you,
Julia