clarification

I arrive Beijing on a Friday at 2 p.m. my 72-hour Free time clock begins after midnight going to Saturday. So, please confirm, I would have Saturday (24 hours) Sunday (48 hours) and Monday (72 hours). I must depart then before 23:59 Monday (the end of the 72-hour period), correct?

The time on Friday would not count against me, correct?

And, I could not stay 72 hours -- Saturday to Monday -- and then depart before noon on Tuesday, correct?
Just want to be sure.