Graymist, on my Cat engine when the mechanic hooks up his computer to it, the idle time is given in hours, minutes & seconds & also as a percentage of total run time. There is also a wealth of othe information about your engine as well... such as over rpm & speed, how long you spend in each gear & at what rpm you shift at. It can really help if you are looking for information on how to improve your fuel economy for instance.
Next time you are in the shop, ask them for a printout on your engine, it takes only a minute or two & most places will not even charge for doing it, especially if you have a good rapport with the place you go.