What is tacho time?

Tacho time is engine hours counted by a recording tachometer, which runs in proportion to engine RPM, so it counts a full hour only at the RPM it's calibrated for. At lower power it counts slower than the clock, which is why many clubs bill by tacho time and why it usually reads less than a Hobbs meter for the same flight.

How RALPH handles it

In RALPH a tacho is one kind of meter an aircraft can declare, never a column every aircraft must fill. An aircraft with a tacho records a tacho reading on every logged flight, beside any other meters it has, and the meter’s reading comes from that trail and its audited adjustments.

Questions, answered

Do all aircraft have a tacho?
No. Many piston aeroplanes do, but plenty of aircraft have only a Hobbs or another hour meter, and some helicopters use a meter started by raising the collective.