There's the leap year and then there's the leap second. Because the Earth is slowing down, timekeepers at the U.S. Naval Observatory, which maintains the Defense Department's Master Clock, and the National Institute of Standards and Technology, which maintains the official U.S. time, have to add a second on to the nation's atomic clocks every so often. It turns out it's that time again. The additional second will be added on Dec. 31 at 11:59:59 pm. The last time timekeepers had to add a second was on Dec. 31, 2005.
A more in-depth explanation for the leap second can be found here.
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