you might have to enter the BIOS, usually the second option (probably 'F2' in your case). Then set time and date to the right values. Exit the BIOS with the option 'Exit saving changes' or 'Save changes and exit' or similar. See if the problem still persists, especially after the computer has been off power for a couple of hours.
If you continue getting that message then probably something's wrong with your motherboard, leaving you with two options...
- live with the problem
- replace the motherboard
But first try what I said above. Chances are that it solves your problem. Lets not assume the worst for a start...