I'd like to add that there are seemingly innocent things that people do on their computers that change the system clock without their realizing it. One very common way is clicking that handy little clock in the corner of the screen to check a date on the calendar. Before you know it, you've clicked OK and your system clock has been set to some other date in the past or future. It's easy to do, and you have to be careful.

Windows and Mac OS X will also update an inaccurate system clock using an Internet time reference, effectively changing the clock for you. You probably wouldn't notice that adjustment, either.