still confused, when do the 72 hours begin?

Is the 72hr visa commencement time, the same at all airports? I will fly Hong to Guangzhou arriving Tuesday at 9.30am. I plan to leave on Saturday to Manila at 7am. If the 72 hours begins at 00.00 on Wednesday as stated, then this will be allowed. But if I plan to leave on Saturday, and it starts from my arrival time, I will have overstayed. I just want to ensure that the 00.00 start of 72 hours on the day following my arrival is accurate.