| There is an onboard battery that keeps the BIOS data when the PC is powered off. It keeps the internal clock chip running also.
On the motherboard, the battery will be silver and coin shaped. It should have some markings on it, such as Maxell, etc. The battery type is CR2032 and is 3.0V. Most battery stores will stock this.
Another item you might want to look at is to see if the windows time is being sychronized with a time server.
Double-click on the date/time to bring up the Date and Time properties, and select the "internet time" tab. |