from when do they start/end counting time for the 72 hr visa?

Hello,

I have a question regarding the 72hr free visa. I'm scheduled for a flight from ho chi minh, vietnam to bejing on the 9 of may that will arrive at 11.30. I will then fly out from Beijing on the 12 of may at 13.50 to sweden. That is in total ca 75 hr, so my question is when dose the visa time start from, my arrivel time or when i get my visa? And when dose the visa expire, when the scheduled fight is supposed to leave or when i'm back at the airport and have passed through the passport/visa control? I mean i still have to be at the airport a couple of hours before the fight anyway or can i hang around the airport before i get my visa.
Regards Gabriel