You need something that works at the BIOS level, this is totally independent of the OS - simply part of the motherboard. You could create a plug-in card with a GPS or atomic clock radio link (for the accurate time) with an EPROM on board which intercepts the BIOS (just like a video card does). During power-up the program in the EPROM could read the time from the accurate clock system, and write the details to the motherboard clock. If you want it to be constantly updated you could have it do so by regular interrupts - possibly at the BIOS level (I'm not that familiar with PC BIOS).