Not sure, normally if it won't keep time its usually the Cmos battery and it would have to be a reasonably old PC cos they don't often fail. But I've never heard of the date and time coming back later on by itself.
Sorry just reread after woodchips post, its not changing back to the correct time its losing the settings again. Its more than likely the battery and the error message is because something in the BIOS is losing its setting at the same time.
Yep and the cheapest place to buy one is usually the likes of boots, I said usually as one computer retailer wanted to charge me £8 for the battery, needless to say I never bought it, got it at boots for £2.99 I think it was. Just take the old one out and look at the number on it.