Depends on your installation.
Some older installations are connected to the tachometer in such a way as to count revolutions, so the meter reading is a theoretical one, based on WOT hours, not on time.
Most are just powered by the ignition switch, so count time the switch is in the "on" position.
Use a multimeter on the existing hour meter to see where it gets power and connect a new one the same way.
If tying into the ignition circuit, you need to be in the habit of leaving it off, otherwise you record hours that the engine was not running at all. (that explains how my Port engine hours climbed past my Stb engine without either being in use)