The VDO Tach unit shown has a digital readout for calibration and eng. hr functions. The engine hours will only post when there is rpm
inpunt. Not with the key on and not running. I think the clock is wrong.
VDO once made Tachs that the hour meters were based on fixed rpm
. Example: 1800 rpm for 1 hour showed 1 hr. At 900 rpm for 1 hr it would show 1/2 hr. This was used to get a better picture engine ware which is not based on time but piston travel. The normal running RPM (ie. 1800) was determined by the manufacturer. This practice was dropped in the early 80's.