Don’t screw up your New Years Eve countdown; 2016 will be lasting a little bit longer than you’re used to. You see, the world is about to experience its 28th leap second.
For you that means that after 11:59:59 pm on December 31, the clock will not tick to 12:00:00 am on January 1. Instead, time will officially read 11:59:60 before incrementing to 12:00:00, the result of a quirk in global timekeeping that’s more useful to astronomers than everyday people. We call it the leap second.
I won’t go into all the nuances of the leap second here; for that you can read my post—The origin of leap seconds, and why they should be abolished—from June 2015. That article discusses why these quirks of official time exist and the (misplaced) wisdom of having leap seconds at all.
Here’s the summary: The Earth doesn’t spin at a constant speed let alone at the perfect rate of once every 24 hours. Recently, one Earth rotation has been taking about 24:00:00.01. So to maintain the millennia-long convention of keeping time tightly linked to the position of the sun in the sky and to make the work of astronomers marginally easier, we—as a global society—simultaneously adjust our clocks.
Leap seconds, unlike leap days (the day added to the calendar every four years) and annual daylight savings adjustments, are inserted at the exact same moment worldwide, so for anyone in the UTC±00:00 time zone or west of it—e.g. London, New York, Caracas, and most of the Western Hemisphere—will have one extra second in 2016. Paris, Moscow, Tokyo, and most of the Eastern Hemisphere will have one extra second in 2017.