Why my computer date and time is changing always?

If your computer time is automatically changing every day or anytime, the CMOS battery of your computer needs to be changed.  You can replace it by opening your computer case or ATX cabinet and you will see the CMOS battery as below

 

It will look like a big watch battery. You can remove it and replace it with a new one. The voltage value and other values of the battery will be specific and so you can take the battery to the store or watch shop where you wish to purchase it. After replacing the CMOS battery, restart your computer and if your computer shows any error restart the computer again. Now you need to set the date and time.

Note:

 Replacing the battery will reset your BIOS as well which is good. Click here to know how that happen.