Timekeeping can be as simple as counting "one Mississippi, two Mississippi . . ." prior to blitzing in backyard football, or tracking the back-and-forth swings of a pendulum in a grandfather clock. In both cases, the trick is counting the intervals of something that occurs repeatedly with as little variation as possible. A pendulum swing, say, or a 'Mississippi' just about equates to a second, the unit of timekeeping that as we know comprises minutes and hours. But even the best mechanical pendulums and quartz crystal-based clocks develop discrepancies. Far better for timekeeping is the natural and exact "vibration" in...