I would suspect the CMOS battery is dead. It is a simple matter to replace
it in most modern machines. Generally, it is about the size of a US nickel
and can be easily seen on the motherboard. Replacements can be found in
nearly any store that sells watch or camera batteries. Changing the battery
can cause the loss of any custom BIOS settings, but I suspect you are
probably using defaults anyhow or you would have noticed other system
problems.