It depends on who/what/where and how.
The mob I used to work for had us log off/on chock times as well as wheels off/on times.
Our flight time limitations were calculated on number of sectors, time of commencement of duty. Usually 1 hours prior to engine start to 15 min after on chocks. Usually this corresponded to sign off time but occasionally we'd have to position via taxi or jet.
Duty pay was worked out on sign on/sign off time.
Maintenance is often worked out on Wheels off/on times. It depends on what aircraft etc.