On New Year’s Eve 2016, the world’s timekeepers will extend the year by exactly one extra second.
Official clocks will hit 23:59:59 as usual, but then they’ll say 23:59:60, before rolling over into 2017.
This is known as a “leap second,” and timekeepers slip them in periodically to keep our clocks in sync with the Earth’s rotation.
They do this because it technically takes Earth a bit longer than 24 hours to complete a full rotation — 86,400.002 seconds rather than 86,400. So in order to keep our clocks matched up with solar noon, when the sun is highest in the sky, a leap second gets added every few years.
In fact, this adjustment is quite common: Since the practice began in 1972, fully 27 out of 44 years have included leap seconds. The last one occurred on June 30, 2015.
In practical terms, this won’t affect most people’s lives very much.
Though a previous leap second in 2012 caused a glitch in the software running Reddit, Gawker, and other websites, most systems were far better prepared for the leap second in 2015.
The reasons for the leap second, though, are pretty fascinating — and they reveal some underappreciated facts about the difficulty of precise timekeeping on this spinning chunk of rock we call Earth.
When does the leap second happen?
It depends what time zone you’re in. For those living in Coordinated Universal Time — the world’s official time standard, based off atomic clocks and used to calculate the times around the world — the leap second will happen on December 31 just after 23:59:59. Clocks will move to 23:59:60 before moving on to 00:00:00 the next day.
Revelers in the United Kingdom will have to adjust their New Year’s countdown accordingly.