It is the parking brake which determines the times via ACARS and this also determines the block time for which the crew is paid. Once we sat at the gate in EWR for 2+ hrs (2007 timeframe) and I noticed via my phone it said we had departed on time. In talking to the crew in the back galley they confirmed the brakes were released. Listening to channel 9 at that time was entertaining since most flights calling up delivery and asking for release were told to be patient and sometimes ATC would start to count their position when asked which was usually over 75.

Almost everytime I have flown WN there is someone waiting on the jetway when the plane pulls into the gate. Just one of the small differences between how their staff are trained.