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WASHINGTON – Already a leap year, 2016 will drag on a bit longer. International timekeepers are adding a leap second at year’s end.

Because Earth is slowing down a tad, occasionally timekeepers insert another second or two to match Earth’s rotation and the precise atomic clocks. U.S. Naval Observatory’s Geoff Chester says the tides’ interaction with the moon and other factors, including warmer, denser waters from El Nino, cause Earth to take longer to go full circle each day.

READ MORE: Leap Year 2016: What is it and why do leap years exist?

Timekeepers announced Wednesday that on Dec. 31, at 11:59 p.m. and 59 seconds Universal Time (6:59 p.m. Eastern Time), the next second will become 11:59:60. The new year won’t start for another second.

This will be the first leap second since July 1, 2015.

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READ MORE: 2015 will have 1 extra second, which may cause problems for the Internet