It's the CMOS battery. The CMOS stores a few key bits of data needed for booting the system, one of them is the time and date, and there is a small coin cell battery on the motherboard that is responsible for maintaining this data. If you removed that battery for a minute, and then started your system, you'd notice the time and date would be wrong.
I have noticed that right after I reformat my computer and before I connect my modem, the correct local time is already shown on the taskbar. Just wondering how it does that? Is ?windows? somehow picking the time off the power plug? LOL. Just curious.
Thanks for any insight,