In about 10 minutes, time will once again stop for just a moment as clocks go from 23:59:59 UTC to 23:59:60 before slipping to midnight:
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.
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."
2016 was already a leap year in the Gregorian calendar, will be one second longer still. So if you've felt like 2016 was the longest year ever...you're technically correct.