In the flying school environment you'll find a great deal of misplaced obsession with regard to how to record time on the MR. Usually this is part of the overall obsession with logging time in general since that's how your flying school and instructor gets paid. Also if you cross hire an aircraft the temptation may be to under record time since that will benefit you when it comes to paying the bill. These aircraft are usually fitted with an OPTIONAL timer. This could be tacho time or a timer which runs when there is oil pressure or a timer which runs when the aircraft is airborne. Only one of these figures is the correct time to record on the MR. Outside this arena in GA or the airlines you'll be lucky to find anything in the cockpit which records TOTAL airborne time. Sometimes there is a timer connected to an airswitch or squat switch which can be referenced to record time for the MR. However most operations I've been involved with simply rely on the pilot noting the takeoff time and the landing time, the difference between the two is noted for each flight and the the total is written on the MR at the end of the days flying. All legal and all as per the company operations manual.