ummm.... do you mean the time that you are on a PC? Or do you literally mean the date/time that is set on the PCs? If you mean the time you spent on the PC, as Looney said, probably radius. If you mean keeping the date/time correct on all the PCs, probably "Windows Time service" or possibly NTP (especially if they have a mix of linux/unix and windows systems, NTP will more likely be used).