when I was in Win98, I had a different program which told me the offset between my clock and the atomic clock, and asked me if I wanted it corrected now.
As I remember it only drifted a few seconds a week, so once a month or so, I'd correct it. Now, it happens automatically every morning, so I'm not sure exactly how much it needs to correct, but I do know I'm within a second or so all the time...which allows me to re-set clocks exactly in case of a power failure, and to know exactly when to watch for the news.... (Programs like NBC news generally hit the hour VERY precisely).