Depends exactly what you are looking at
For your personal log book time from rotors start to stop, you are PIC so don't leave the machine with rotors still spooling down!
For ac this has many different ways of being recorded
1. Hobbs meter. Records in .1 hours ( 6 mins) usually taken from gearbox pressure
2. datcon usually taken from a squat switch or collective switch new 300's and R44's respectively.
3. Good old fashioned watch
Whichever way you do it there is an element of inaccuracy or deliberate falsification.
told by Schweizer that they expected times to be from moment ac leaves ground to moment back on hence their fitting squat switches
hope that helps