If your computer is <5 years old, chances are it's the BIOS battery dieing. The BIOS uses a small coin cell battery to keep the time and date when you power off the computer, if it's a desktop, just simply open the case and locate it on the motherboard (It's usually near the PCI slots). If you have a laptop, It's sometimes under the main battery, or under the RAM or hard drive. If it's nowhere easily accessible, you may have to take your entire laptop apart, or pay some one a ridiculous amount to do so. If it's a modern computer, it could just be a software problem. Try entering BIOS next time you start up and see if the problem persists. If it does, it's definitely the battery. If not, it's a software problem and there isn't much I can suggest for that.