Why do the time on my computer changes for no reason?

The reason is your cmos battery is about to die. All desktop PCs use a small lithium battery, similar to those used by hand-held calculators to keep time and date info. Once this battery runs low on voltage, your date and time will be shown incorrectly.

The solution: unplug your PC, open the case and carefully extract the battery (use a small flashlight). Buy a new one of the same number (cr2032, cr2025, etc.) and install it the right way. Don't worry, it will fit only one way. Now turn your PC on and set the time and date and that's it, no problem with that for a good few years. Hope this helps.