72 hour visa issue in Beijing

Hi!
Me and my husband were planning on using the 72 h free visa in BJ.
We will fly from phnom penh to BJ with a transfer in shanghai:
From phnom penh kambodja 08/02 2014 00:15 to shanghai 04:55. We will stay inside of the airport.
Then 13:00 thesame day we will fly from shanghai to BJ 15:20.
11/2 14:20 we will fly from BJ to munchen.

My question is. When does the 72h start counting? Is it from when we transfer in shanghai airport and wait for thenext flight inside the airport or is it from when we stepoutside of BJ airport?
Do we haveto get visas?