Sounds like the CMOS battery on your motherboard has died. Make sure your computer is off and unplugged. Open the case and look for a shiny chrome disc about the size of a US quarter. It may be flat against the board or standing up in a slot. At any rate remove it. The part number will be on the back. Something like CR2132. These are sold just about anywhere that sells batteries.
If you want to know the technical stuff, you can Google "CMOS battery".