Home   Time Zones   Leap Second

What's a Leap Second?

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

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.

Illustration image

Atomic clocks are slightly too accurate.

Leap seconds are added to realign clocks worldwide with the Earth's rotation.


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.

When the difference between UTC and UT1 approaches 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 with atomic time.

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

The Science Behind Leap Seconds

Atomic Time Too Accurate

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

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 26 leap seconds have been added. This means that the Earth has slowed down an additional 26 seconds compared to atomic time since then.

However, this does NOT mean that the days are 26 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.

Check our Time Zone News for updates about leap seconds.

Topics: Timekeeping, Clocks, History


Leap Seconds

  1. What's a Leap Second?
  2. How Leap Seconds Work
  3. The Future of Leap Seconds
  4. How Long is a Day on Earth?
  5. International Atomic Time (TAI)
  6. How Do Atomic Clocks Work?

Go to Library

International Atomic Time (TAI)


Tuesday, January 17, 2017

TAI – Currently 37 seconds ahead of UTC

Coordinated Universal Time (UTC)


Tuesday, January 17, 2017

UTC is the common time standard across the world

More about UTC

You might also like

Why Do We Have Time Zones?

Which country first adopted time zones? Where and why were standard time zones first adopted and why were time zones created? more

Business watch and one hundred dollar bill

History of DST

Daylight Saving Time (DST) is used to save energy and make better use of daylight. Today, daylight saving time (DST) is used in many countries. more

UTC – Coordinated Universal Time

Coordinated Universal Time (UTC) is the basis for civil time today. This 24-hour time standard is kept using highly precise atomic clocks combined with the Earth's rotation. more