The FMS generates the event based on the squat switch, which tracks weight off wheels (OFF) and weight on wheels (ON). This is what then generates the datalink message. Any latency in the onboard systems could cause the FMS and datalink to be one minute off.

Out and In times are based on brake set and brake release, in combination with the door being opened, but that is only for aircraft equipped with these sensors and setup to transmit those message on the ground. Not many Business aircraft are equipped in such a manner. The only other way to get Out and In, is from submitting times via Post Flight on the ARINCDirect FOS Mobile app or someone entering them via the flight following feature within the core FOS system.