.
The tacho clock is just a convenient method of recording time; what ever time you want it to be. It is an American invention for convenience.
Before American aircraft/helicopters became the the majority; most British or European aircraft had no time recording clock fitted !
Aircraft maintenance and pilot logged flight time is based upon honesty.
For maintenance: Engine and airframe time is based upon wheels' off' to wheels 'on'; simple as that. (for simplicity - operators may fit an air switch to start recording at a pre determined airspeed - for helicopters that may be a torque setting or similar)
Pilot log book time: Based upon Start taxi to finish taxi at conclusion of each sector or flight. In commercial Operations where flight and duty time may be an issue over a 30/90/365 day period, CASA may accept via the approved Ops Manual; wheels on to wheels off + 5 minutes or 0.1 if logging digital time.
That's it ....
If you see aircraft sitting idling whilst conducting maintenance runs; waiting for take-off etc, none of that time is recorded for maintenance.
Flight schools need to make a dollar to cover overheads, power, light, air-con, admin staff, rent etc so they may well fit a VDO or Hobbs meter which would be oil pressure operated at engine start. They would bill the student or client by this method as they require money to make the business work.
If you check out the big name flight schools; you will find two Hobbs meters; one is for engine running time (billing hours) and the other is for airframe/engine maintenance. It is quite possible to have a C172 in a flight school with say 2000 hrs total time for engine/airframe maintenance; but showing 3000+ hours for billed out engine running time to the student or private hire. In this example the 3000+ hours has nothing to do with eng/airframe maintenance.
If you are a responsible owner/operator you do not require ANY time clocks in your flying machine. IF you carry out commercial operations the contract client may require a time clock to verify billed time is indeed maintenance time, otherwise you would have operators idling for 20 or 30 minutes to pad the billing time.
If in doubt refer to the CASA regs..