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About Leap Seconds

Did you notice? The last leap second was added at 23:59:60 UTC on June 30, 2012. See table below for detailed information.

Illustration image

Earth is too slow for our highly precise atomic clocks


What are leap seconds?

They last only a heartbeat and go unnoticed by most - but without leap seconds our clocks would run too fast.

About every one and a half years, one extra second is added to Universal Coordinated Time (UTC) and clocks around the world. This leap second accounts for the fact that the Earth's rotation around its own axis, which determines the length of a day, slows down over time while the atomic clocks we use to measure time tick away at almost the same speed over millions of years.

So, leap seconds are a means to adjust our clocks to the Earth's slowing rotation.

More details...

How many leap seconds have been added so far?

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 2012

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

UTC DateUTC TimeLocal time world-wide
2012-06-3023:59:57Corresponding times
2012-06-3023:59:58Corresponding times
2012-06-3023:59:59Corresponding times
2012-06-3023:59:60Leap second added
2012-07-0100:00:00Corresponding times
2012-07-0100:00:01Corresponding times
2012-07-0100:00:02Corresponding times

Can we live without leap seconds?

When are leap seconds added?

Leap seconds are inserted at the end of the last day in June or December. When that is the case, UTC ticks from 23:59:59 to 23:59:60 before reverting to 00:00:00 (in the 12-hour format, this corresponds to 11:59:59 pm - 11:59:60 pm - 12:00:00 midnight). When that happens the last minute of the month has 61 instead of 60 seconds.

The last time a leap second was added to UTC was at 23:59:60 UTC on June 30, 2012 (see table). The difference between UTC and the International Atomic Time (UTC-TAI) from July 1, 2012 is -35 sec.

Who decides when leap seconds are added?

The International Earth Rotation and Reference System Service (IERS) 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.