Be ready to have your New Year’s celebrations last a second longer. Literally. Timekeepers at the International Earth Rotation and Reference Systems Service (IERS), which keeps track of the world’s time, announced yesterday that an extra second will be added on December 31st. Around midnight, the clocks will read 11:59:60 instead of 11:59:59.

Leap seconds are added once in a while to ensure that the Coordinated Universal Time (UTC), the global measure of time, is adjusted to follow the variations in Earth’s rotation. Earth’s spin is not constant: the Moon’s gravitational pull and other factors like particularly strong earthquakes can affect how long our planet takes to rotate on its axis. Leap seconds ensure that atomic clocks can match our planet’s rotation, which is gradually slowing down.

The Earth's rotation is gradually slowing down

Because leap seconds can create problems with computer systems that weren’t designed to account for them, some are calling for leap seconds to be abolished. But that probably won't happen anytime soon. In November, representatives at the World Radiocommunications Conference in Geneva decided to postpone a decision on whether to keep leap seconds to 2023.

To create less confusion with the one-second adjustments, the IERS announces leap seconds always six months in advance, for either June 30th or December 31st. Since 1972, the IERS has added 26 leaps seconds. The most recent one was on June 30th, 2015.