Share Charles Krupa/APYour obligatory shot of a clock — with this crucial reminder: The ineluctable march of time will dally just a little bit when it bids farewell to 2016.Here’s a timely reminder for all you would-be revelers out there: Be careful with your countdowns this New Year’s Eve. There will be a little extra time to bask in the glow of a retreating 2016 — or curse its name, as the case may be.Whatever your inclination may be, one thing is certain: Before the year is out, the world’s foremost authority on time will be adding one more second to the clock.In a bulletin released this summer, the International Earth Rotation and Reference Systems Service, or IERS, said it would be necessary to introduce a “leap second” at the end of December. Timekeepers use this added second much as leap years are used — to bring the world’s atomic clocks in sync with the Earth’s own distinctive rhythm, which in this case is determined by its rotation.This leap second isn’t the first. Since 1971, the world has added leap seconds with some regularity — typically every two to three years — and the latest leap second was added only last year, in June.Why do these leap seconds keep cropping up? Peter Whibberley, a research scientist with the U.K.’s National Physical Laboratory, explains it this way:“Atomic clocks are more than a million times better at keeping time than the rotation of the Earth, which fluctuates unpredictably. Leap seconds are needed to prevent civil time drifting away from Earth time. Although the drift is small — taking around a thousand years to accumulate a one-hour difference — if not corrected, it would eventually result in clocks showing midday before sunrise.”Recent research appears to back up Whibberley’s assessment. According to a study published earlier this month in the Proceedings of the Royal Society A, Earth’s rotation has slowed about 1.8 milliseconds per day — which means the solar day itself has lengthened, little by little. The researchers based this assessment on records dating back to 760 B.C., long before the implementation of the precise atomic clocks.The Los Angeles Times broke down the findings: “If humanity had been measuring time with an atomic clock that started running back in 700 BC, today that clock would read 7 p.m. when the sun is directly overhead rather than noon.”Still, however useful they may seem, these little extras can occasionally cause some big frustrations, as NPR’s Geoff Brumfiel notes:“Leap seconds have crashed airline reservation systems. They’re believed to have briefly shut down Russia’s GPS satellite system, and there’s potential for even greater mischief as things like financial trading become ever more precise in their use of time.”And those issues have earned the humble leap second its fair share of critics, who argue that it is standing in the way of the precision promised by atomic clocks. These critics would prefer the steadiness that may come with unmooring the world’s measurement of time from its astronomical inspiration.The debate has persisted for years, leaving the leap second with an uncertain fate. The United Nations announced last year that it will be postponing its decision on whether to keep the leap second around, saying that “further studies are required on the impact and application of a future reference time-scale.”The organization is not expected to return to the decision until 2023.All this is to say: Whether you like it or not, the leap second will be sticking around a while longer — and so will 2016, even if only by a single second.Copyright 2016 NPR. To see more, visit http://www.npr.org/.