Your logged time as the pilot flying the plane should be the same as the time the plane's flying (flight) time. Therefore I do, and suggest to you, that you use the time of day wheels off, to wheels on as your "air time". It seems customary to add 0.2 hours to that, so as to determine "flight time" for the aircraft (taxiing etc.). Hoobs and tach time are not really that accurate, and I would not want to depend upon them. I have certainly known Hobbs to not work, and tachs be quite in error.
I have never known this "air time + 0.2 hours" formula to not be accepted for logging piloting time, though others may do it differently.
Those decimals of an hour won't seem so important to you later in your flying career...