Originally Posted by
Kelron
Turning it off at the mains will cause this, eventually. If you leave it plugged in it will receive enough power to keep the time. If it's not receiving power from the mains it's draining the battery, and it won't be keeping time correctly overnight. So you're turning it on the next morning and it's still showing the time from the previous night.
If you replace the battery while the system is on at the mains (with the PC off, of course), you won't lose the settings. Obviously you wouldn't normally want to work on a computer with the mains on, but you will lose your settings otherwise.