Just in time for a long holiday weekend for the U.S., clocks will receive an extra second today (June 30). The reason - "The solar day is gradually getting longer because Earth's rotation is slowing down ever so slightly," says Daniel MacMillan of NASA's Goddard Space Flight Center in Greenbelt, Md."
NASA Explains Why Clocks Will Get an Extra Second on June 30 tells that Earth's rotation is slowing down because of tidal forces between the Earth and the moon. Every 100 years, the day becomes 1.4 milliseconds longer. Over time, those milliseconds add up. In fact, at the time of the dinosaurs, the day was about 23 hours long.
Because the Earth is not a very reliable time keeper, scientists changed the definition of a second to a cesium standard, or atomic clock in 1967. NIST-F1 picture above creates a cesium ball of atoms that is excited by a microwave signal. A laser then measures the frequency of the photons produced by the cesium. Since that is precisely known as 9,192,631,770 Hz, it permits the uncertainty of the clock to be less than 3x10-16. That means it will not change a second in 100 million years. (Click on the NIST-F1 link for the complete description.)