I agree with Neo. I also think there's a utility you can download and install that automatically updates your clock via the internet. I've never used it though, and I'd have to find it with Google...
Are you sure that you have it set to the correct time zone? I made the mistake once of setting it to "Western US" time zone instead of EASTERN where I am located, and my clock would always lose an hour randomly. Check real quick to make sure it is in fact correct.