There is a "CMOS clock" in your computer which is powered by a tiny battery.
As long as the battery is good, this clock keeps the correct time,
and each time your computer is restarted, Windows reads its initial time from
the CMOS clock. However, while Windows is running, it keeps track of the time
on its own without continuing to check the CMOS clock, and keeping track
of the time is not the only thing Windows has to do. The busier your system gets,
the more likely it is to lose time. Generally, the longer you use your computer,
the further behind it gets. You may also need to update your bios.