The CMOS battery enables the computer to remember the date, but when the battery goes flat the computer will revert back to the date of manufacture. Changing the CMOS battery can be difficult on some models. Many laptops use a special type of battery with leads soldered onto it, but soldering the leads onto a battery is very dangerous. This video shows how to convert a regular coin battery to a battery that can be used in most laptops for a fraction of the price. How to change the cmos battery videos can be seen here- https://www.youtube.com/watch?v=cg_wvp1YdSI https://www.youtube.com/watch?v=aUk1UxYOpW4