TWOV Stopover BOTH ways?

1) Can you stop over without a tourist visa in Beijing for 72 hours on the way to AND from Korea? My flight to and from Beijing is direct.

2) How are the hours calculated? I've read on some websites that Beijing airport immigration calculates by starting the first day at 00:00 the day following the arrival, but I really want to be sure about this so I don't have problems. I'm arriving on 9 April at 6:00 and we leave for Korea at 1255 on the 12th. Just a few hours over the strict arrival/departure flight times, but it would be acceptable if the day started at midnight on the 10th. I'm really concerned about my airline not allowing me to board the plane, because I go over just a few hours. I'm taking 2 separate airlines. Is there a way to verify this without calling the airport in Beijing?

Thank you!!