So -- When do we actually have to have our departure flight? What does "exit" mean -- clear customs?

e.g.
LAX -> HKG -> CAN -- Arriving 12/23 @ 11:35. Apply for 72HR TWOV. Clock starts 12/24@00:01.
CAN -> SIN on 12/27 @ 03:45 -- technically 75 hours and 45 minutes.

If I enter the airport -- as in, get my passport stamped exit -- before midnight, am I good to go?