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What are Leap Seconds?

A leap second will be added on June 30, 2015 23:59:60 UTC.

Illustration image
Atomic clocks are slightly too accurate.
Leap seconds are added to realign atomic clocks with the Earth's rotation.
©iStockphoto.com/kvkirillov

A leap second is a second which is added to Coordinated Universal Time (UTC) in order to synchronize atomic clocks with astronomical time to within 0.9 seconds.

Why Do We Need Leap Seconds?

The reason we have to add a second every now and then, is that Earth's rotation around its own axis, is gradually slowing down, although very slowly.

Atomic clocks however, are programmed to tick away at pretty much the same speed over millions of years. Compared to the Earth's rotation – which determines the length of a day – the atomic clocks are simply too accurate.

Exactly how do Leap seconds work?

How Often Are Leap Seconds Added?

Did you notice? The last leap second was added at 23:59:60 UTC on June 30, 2012.

Since 1972, a total of 25 seconds have been added. This means that the Earth has slowed down 25 seconds compared to atomic time since then.

This does not mean that days are 25 seconds longer nowadays. Only the days on which the leap seconds are inserted have 86,401 instead of the usual 86,400 seconds.

Leap Second 2015

Click on Corresponding times to find out when the leap
second is added to the time at your location.

UTC DateUTC TimeLocal time world-wide
2015-06-3023:59:57Corresponding times
2015-06-3023:59:58Corresponding times
2015-06-3023:59:59Corresponding times
2015-06-3023:59:60Leap second added
2015-07-0100:00:00Corresponding times
2015-07-0100:00:01Corresponding times
2015-07-0100:00:02Corresponding times

Can we live without leap seconds?

36 Seconds' Difference

The difference between UTC and the International Atomic Time (UTC-TAI) after the next leap second has been added on June 30, 2015, will be 36 sec.

Who decides when to add leap seconds?

The International Earth Rotation and Reference System Service (IERS) in Paris, France observes the Earth's rotation and compares it to atomic time. When the difference between the two approaches 0.9 seconds, they order a leap second to be added worldwide.

Check our Time Zone News for updates about leap seconds.

Topics: Timekeeping, Clocks, History

In this Article

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Leap Seconds Library

  1. How do leap seconds work?
  2. The Future of Leap Seconds
  3. Intl Atomic time (TAI)

What's a Leap Second?



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