Source of information?

Hello! I've been denied boarding following your advice, which matches the TIMATIC - IATA system. I fulfill all of the conditions of the 72 hr visa according to you, but there was controversy in between the airline staff on when the 72 hrs would start counting and the staff said that the Chinese at PEK would not allow more than strict 72hrs from landing-time to departure-time. Hence, I was denied.

I know I'm not alone, so I've been trying to search an official website with this info, but I have not found anything that says that we can stay longer than the strict 72 hrs. Could you share your source? A source in Chinese would be preferable. Thank you.