When does the Transit Visa start?

Hello,

I am traveling to Beijing and technically from arrival flight to departure flight I would be in Beijing for 78 hours instead of 72. When does the transit visa time technically start? Does it begin at the time your visa gets stamped into your passport? Or does it start the following day? I have read that there are different rules for different airports. So at the Beijing airport would it be possible for me to get a transit visa even though technically I'm 5 hours over? If it begins once the visa is stamped into my passport is it okay if I stamp out at the 72-hour mark and just stay in the airport and wait for my flight?