The tach(ometer) timer is an instrument used in aviation to accumulate the total number of revolutions performed by the engine.
The unit of measure is equivalent to the number of hours of running at a certain, specific reference speed of rotation.
[1] The tach timer integrates over time the instantaneous rotation speed displayed by the tachometer.
The displayed number is incremented by one if the engine is run at its reference speed for one hour.
If the reference rotation speed is 2400 RPM then the tach timer records The tach timer is usually used to schedule engine maintenance, although it is just an approximation of "Time in service" which is used to time and schedule aircraft maintenance.