Leap Seconds FAQs

A leap second is a second added to Coordinated Universal Time (UTC) in order to keep it synchronized with astronomical time. UTC is an atomic time scale, based on the performance of atomic clocks that are more stable than the Earth's rotational rate. Astronomical time (UT1), or mean solar time, is based on the rotation of Earth, which is irregular.

There are two main reasons that cause leap seconds to occur. The first is that the duration of the atomic second was measured and defined by comparing cesium clocks to the Ephemeris Time (ET) scale, an obsolete time scale that defined the second as a fraction of the tropical year. The duration of the ephemeris second was slightly shorter than the mean solar second and this characteristic was passed along to the atomic second. If the atomic second had been defined with respect to the mean solar second, it is likely that leap seconds would have been required much less frequently. The second reason for leap seconds is that the speed of the Earth's rotation is not constant. It sometimes speeds up, and sometimes slows down, but when averaged over long intervals the trend indicates that it is gradually slowing. This gradual decrease in the rotational rate is causing the duration of the mean solar second to gradually increase with respect to the atomic second.

For information about past leap seconds and the measured differences between UTC and UT1, please see the NIST Time Scale Data Archive.

Leap seconds are added in order to keep the difference between UTC and astronomical time (UT1) to less than 0.9 seconds. The International Earth Rotation and Reference Systems Service (IERS), measures Earth's rotation and publishes the difference between UT1 and UTC. Usually leap seconds are added when UTC is ahead of UT1 by 0.4 seconds or more. At the time the corrections started in 1972, a necessary correction of ten seconds was made to UTC, and there have been leap seconds about every year and a half (on average). The current difference between UTC and UT1 is shown in our leap second archive web page.

Leap seconds have always occurred at the end of December or the end of June, on the last second of the UTC day. The designation of the sequence of seconds is:

23h 59m 59s
23h 59m 60s
00h 00m 00s (the beginning of the following date)

So far, there have only been positive leap seconds. However, there is a provision for negative leap seconds if it becomes necessary due to changes in Earth's rotation. The designation of the sequence to remove a second would be:

23h 59m 57s
23h 59m 58s
00h 00m 00s (the beginning of the following date)

By keeping Coordinated Universal Time (UTC) within one second of astronomical time, scientists and astronomers observing celestial bodies can use UTC for most purposes. If there were no longer a correction to UTC for leap seconds, then adjustments would have to be made to time stamps and legacy equipment and software which synchronize to UTC for astronomical observations. However, adding a second to UTC can create problems for some systems, including data logging applications, telecommunication systems and time distribution services. Special attention must be given to these systems each time there is a leap second.

Some proposals have been made to implement larger corrections like leap hours, which would occur much less often than leap seconds. Also, it has been suggested to stop corrections to UTC and publish the growing difference between UTC and UT1. The relatively small community that needs UT1 could then use the published values as a correction. Discussions of the future of leap seconds are ongoing at meetings of the Radiocommunication Sector of the International Telecommunication Union (ITU-R).

Leap seconds and leap years are both implemented to keep our time in accordance with the position of Earth. However, leap seconds are added when needed, based on measurements, and leap years are regularly occurring events based on set rules. During leap years, an extra day is added as February 29th to keep the calendar synchronized with the precession of the Earth around the Sun. Leap years are necessary because the actual length of the year is 365.2422 days and not 365. The extra day is added every four years to compensate for most of the partial day. However, this is a slight over-compensation, so some century years are not leap years. Only every fourth century year (those equally divisible by 400) is a leap year. For instance, 2000 was a leap year, but 1900, 1800 and 1700 were not.