How wrong is the clock? Is it always off by the same amount? Off by exactly n hours? Or is it just random minutes?
If you don't cut power to your computer completely (yank it out of the wall), then the motherboard battery doesn't matter - it's only there for the moments when the machine fully loses power.
Click on the time in the lower right corner of the screen, you should see a calendar pop up. Choose "change time and date settings". Is your time zone correct? Now click on the"Internet Time" tab - is your machine set to synchronize with time.windows.com?
If those are correct, then there's another reason, obviously. COULD be some malware changing your clock, but not very likely.