Cartridge running time on a VPI SDS

I have heard various people advertising cartridges with a specific number of hours, the time has been registered on a VPI SDS. Can anyone advise me how to do this, no mention of it in the owners manual
The Calibration Mode Info, for the SDS, is available on VPI's website in PDF form. A simple matter of first noting how many hours you currently have on your SDS, before installation of a new Cartridge. Then at a later point in time, just subract those hours, from the new hours registered on the Unit to get actual hours of play from your Cartridge. Mark
The time registered on the SDS indicates how many hours the SDS has been turned on. Does the SDS "know" if a record is playing? What happens if the SDS is ON for several hours or days and no record is playing? Seems like this would give a gross error in actual cartridge use.
Thankyou Mark for your response, quite simple really.