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What Is a Leap Second?

Every now and then, a leap second is added to Coordinated Universal Time (UTC) in order to synchronize clocks worldwide with the Earth's ever slowing rotation.

In Washington DC, the previous leap second occurred on Saturday, December 31, 2016 at 6:59:60 pm.
UTC time was December 31, 2016 at 23:59:60.

Illustration image

Atomic clocks are slightly too accurate.


Next Leap Second

Latest update: In July 2020, the International Earth Rotation and Reference Systems Service (IERS) announced there will be no leap second in December 2020. There was no leap second added in June 2020 either. The next possible date is June 30, 2021.

We will publish the date here as soon as it is confirmed.

Atomic Time vs. Universal Time

2 components are used to determine UTC (Coordinated Universal Time):

  1. International Atomic Time (TAI): A time scale that combines the output of some 200 highly precise atomic clocks worldwide, and provides the exact speed for our clocks to tick.
  2. Universal Time (UT1), also known as Astronomical Time, refers to the Earth's rotation around its own axis, which determines the length of a day.

Before the difference between UTC and UT1 reaches 0.9 seconds, a leap second is added to UTC and to clocks worldwide. By adding an additional second to the time count, our clocks are effectively stopped for that second to give Earth the opportunity to catch up.

Upcoming leap seconds are announced by the International Earth Rotation and Reference Systems Service (IERS) in Paris, France.

Atomic Time Too Accurate

The reason we have to add a second now and then is that the velocity of Earth's rotation around its own axis does not match the speed of atomic time. On average, it is a tiny bit too slow—and it is gradually slowing down, although very slightly.

Atomic clocks, however, tick away at pretty much the same speed over millions of years. Compared to the Earth's rotation, atomic clocks are simply too consistent.

How Often Are Leap Seconds Added?

Before the first leap second was added in 1972, UTC was 10 seconds behind Atomic Time. So far, a total of 27 leap seconds have been added. This means that the Earth has slowed down an additional 27 seconds compared to atomic time since then.

However, this does NOT mean that the days are 27 seconds longer nowadays. The only difference is that the days a leap second was added had 86,401 seconds instead of the usual 86,400 seconds.

Last Leap Second in December 2016

The last leap second was added on December 31, 2016, at 23:59:60 UTC. The difference between UTC and International Atomic Time (TAI) increased from the 36 seconds to the current 37 seconds.

Next Leap Second

Based on current predictions, the next leap second should be added on June 30, 2020. However, since the speed of the Earth's rotation is subject to unpredictable short-term variations, the date may still change. Once it is officially announced, we will publish it in our Time Zone News.

Topics: Timekeeping, Clocks, History