Perhaps you've experienced this before: You arrive for an important appointment, and your watch says you're precisely on time. But much to your dismay, the receptionist remarks that they've been wondering whether you were going to show; she points to the wall clock, which indicates that you're five minutes late.

After you recover from feeling flustered, you probably wonder: Just how in the heck did that happen? After all, you're the super-conscientious sort -- the kind of person who buys a watch and then promptly calls the U.S. Naval Observatory's "correct time" number (202-762-1401) to set it precisely -- down to the second. So how in the world does your timepiece always wind up being a little bit off? And sometimes it's more than just a little bit -- or at least that's what your friends insist. Which leads to another question: Whose timepiece is correct -- yours or theirs? And furthermore, just how accurate can anyone expect his or her watch to be? It's enough to make want to throw up your hands and sing the Chicago's 1969 hit "Does anybody really know what time it is?" (No? You don't feel like singing?)

Well, if it's any consolation, you're hardly the first person to feel bedeviled by personal timekeeping. It's a bewildering thing. But we're going to get to the bottom of it. First, let's take a look at the history of the watch.