A computers CMOS battery has a lifespan of around 5 years before it dies and needs replaced.
Some signs and symptoms that your CMOS battery could be failing or dying or your CMOS battery has already died is failure of your computer to boot up, date and time on computer changes, drivers for your computer might stop working or you get some boot up errors such as “CMOS checksum error” or “CMOS read error“.
The CMOS battery is pretty cheap and easy to replace so it's worth a try to replace the CMOS battery if you're having any of those problems.
But if your time on your computer always changes and is not staying at the correct time especially when the power is disrupted to the computer then it's likely your CMOS battery is either weak or dead.