my clock keeps showing the incorrect time and defaulting to a previous day too. I adjust using day/time and it is fine til i close down. When I next log in sometimes the day is still correct but the time is always wrong.
The computer is in a room which is kept room 65/70 so I know its not "cold".
I believe there is "a battery" but no idea what this is or where it is.
It really does sound like it could be the Cmas battery. They are very easy to change and very easy to get hold of. Last time I bought one - a few years ago now, I got one from Boots. My local computer shop wanted £5 and Boots were selling them for something like £2.50.
If you have a look on the motherboard you will see it.