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Space.com: On 31 December at 23 hours, 59 minutes, and 59 seconds Coordinated Universal Time, one leap second will be added to the world’s official timekeeping system. The reason is to maintain the synchronicity of two types of clocks: atomic clocks, which are based on the vibrations of the cesium atom, and traditional clocks, which are based on Earth’s rotation relative to the Sun. Because Earth’s rotation varies irregularly, the International Earth Rotation and Reference Systems Service, the world's official timekeeper, usually applies a leap second whenever needed to keep the difference between the two time systems from exceeding 0.9 second.
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