The system time may differ from the BIOS time on a Windows Vista-based computer that supports High Precision Event Timer

You use a Windows Vista-based computer that supports High Precision Event Timer (HPET), and the computer runs for several hours. Then, you may notice that the system time differs from the BIOS time by several seconds.

This time difference occurs because the HPET hardware and the Real-Time Clock (RTC) are never completely accurate. The W32Time service synchronizes the system time with a Network Time Protocol (NTP) server to account for any time differences. If there is no active connection to an NTP server, the system time is synchronized with the BIOS clock. If the time difference exceeds 60 seconds, the system time is corrected automatically according to the BIOS clock.