On Dec. 31, 2016, the international time keeping community will tack an additional second, known as a leap second, on to the last minute of the year. To keep the two types of time from veering too far away from each other, the IERS calls for a leap second whenever it appears there will be more than a 0.9 second difference between astronomical time and atomic time. As the “Leap Second” is added simultaneously all over the world at UTC 23:59:59 on December 31, 2016, implying that in India the “Leap Second” will be inserted at IST 05:29:59 on January 1, 2017 (IST being five hours and thirty minutes ahead of UTC). The International Earth Rotation and Reference Systems Service in France tracks the Earth's rotation and announces the need for a leap second. Media playback is unsupported on your device Media caption Rebecca Morelle reports: "It's all because of a very slight wobble in the Earth's rotation"

A "leap second" will be added to this year's New Year's countdown to compensate for a slowdown in the Earth's rotation.
Authorities add a leap second on December 31 to bring UTC more inline with mean solar time.