Utc Leap Seconds

Utc Leap Seconds

This page is updated monthly and contains a table of leap seconds, the current difference between the UT1 and UTC time scales, and the current UT1 - UTC difference that is being broadcast by NIST (called the DUT1 correction).

UTC was already 10 seconds behind TAI before the first leap second was added in 1972. Since then, the Earth has slowed down an additional 27 seconds, and a total of 27 leap seconds have been added (see table). This means that UTC is currently 37 seconds behind TAI (see UTC and TAI clocks). Negative Leap Seconds. Leap seconds were introduced in UTC to keep delta-UT within +-0.9s. TAI-UTC is always an integral number of seconds, and is varied when leap seconds are added (or removed, but that hasn't happened yet) at the end of every year, or every half-year, or every third month, in that order of priority. Since then, there have been 18 leap seconds introduced, such that there is now an 18 second difference between GPS time and UTC time. Instead of reprogramming each satellite’s internal clock with every leap second added, it was decided that an easier method would be to include the current number of leap seconds in the downloaded navigational.

The master clock pulses used by the WWV, WWVH, WWVB, ACTS and Internet Time Service (ITS) time code transmissions are referenced to the UTC(NIST) time scale. Occasionally, 1 s is added to the UTC time scale. This second is called a leap second. Its purpose is to keep the UTC time scale within ±0.9 s of the UT1 astronomical time scale, which changes slightly due to variations in the rotation of the Earth.

See information about why we need leap seconds.

Leap Seconds

Jeff bezos old pictures. Coordinated Universal Time (UTC) is based on International Atomic Time (TAI), but it is adjusted by leap seconds to account for the difference between the definition of the second and the rotation of Earth. This correction keeps UTC in conjunction with the apparent position of the Sun and the stars, and it is the standard used for all general timekeeping applications.
The current difference between UTC and TAI is 37 seconds. (TAI is ahead of UTC by this amount)

No leap second was added at the end of December 2020.

No leap second will be introduced at the end of June 2021.

The first leap second was inserted into the UTC time scale on June 30, 1972. Leap seconds are used to keep the difference between UT1 and UTC to within ±0.9 s. The table below lists all leap seconds that have already occurred, or are scheduled to occur.


All leap seconds listed in the table are positive leap seconds, which means an extra second is inserted into the UTC time scale. The sequence of events is:

23h 59m 59s - 23h 59m 60s - 00h 00m 00s

Leap Seconds Inserted into the UTC Time Scale


Current UT1-UTC values

This table lists the most recent differences between UT1 and UTC. This information is obtained from the United States Naval Observatory (USNO).

Weekly UT1-UTC Values

DateMJDUT1-UTC(±5 ms)UTC(USNO,MC) - UTC(NIST) (±20 ns)
2021-02-2559270-169.34 ms-2.8 ns
2021-02-1859263-171.96 ms-2.0 ns
2021-02-1159246-169.26 ms-1.8 ns
2021-02-0459249-168.85 ms-1.5 ns
2021-01-2859242-167.30 ms-1.9 ns
2021-01-2159235-171.05 ms-3.1 ns
2021-01-1459228-172.88 ms-3.8 ns
2021-01-0759221-174.85 ms-4.2 ns

DUT1 corrections

Leap seconds ensure that UT1 - UTC will always be held within ±0.9 s. The current value of UT1 - UTC is called the DUT1 correction and is obtained from the Bureau International des Poids et Mesures (BIPM). DUT1 corrections are broadcast by WWV, WWVH, WWVB and ACTS, and are printed below. These corrections may be added to received UTC time signals in order to obtain UT1.

The resolution of the DUT1 correction is 0.1 s, and represents an average value for an extended range of dates. Therefore, it will not agree exactly with the weekly UT1-UTC(NIST) values shown in the earlier table, which have 1 ms resolution and are updated weekly.

DUT1 (UT1-UTC) corrections broadcast by NIST

Utc Leap Seconds Gps

Start DateTimeDUT1 Correction
2019-05-020000 UTC-0.2 s
2019-01-170000 UTC-0.1 s
2018-09-210000 UTC+0.0 s
2018-03-150000 UTC+0.1 s
2017-11-300000 UTC+0.2 s
2017-06-290000 UTC+0.3 s
2017-03-300000 UTC+0.4 s
2017-01-260000 UTC+0.5 s
2017-01-010000 UTC+0.6 s
2016-11-170000 UTC-0.4 s
2016-09-010000 UTC-0.3 s
2016-05-190000 UTC-0.2 s
2016-03-240000 UTC-0.1 s
2016-01-310000 UTC 0.0 s
2015-11-260000 UTC+0.1 s
2015-09-110000 UTC+0.2 s
2015-07-010000 UTC+0.3 s
2015-05-280000 UTC- 0.7 s
2015-03-190000 UTC- 0.6 s
2014-12-250000 UTC- 0.5 s
2014-09-250000 UTC- 0.4 s
2014-05-080000 UTC- 0.3 s
2014-02-200000 UTC- 0.2 s
2013-11-210000 UTC- 0.1 s
2013-08-220000 UTC+ 0.0 s
2013-04-110000 UTC+ 0.1 s
2013-01-310000 UTC+ 0.2 s
2012-10-250000 UTC+ 0.3 s
2012-07-010000 UTC+ 0.4 s
2012-05-100000 UTC- 0.6 s
2012-02-090000 UTC- 0.5 s
2011-11-040000 UTC- 0.4 s
2011-05-120000 UTC- 0.3 s
2011-01-060000 UTC- 0.2 s
2010-06-030000 UTC- 0.1 s
2010-03-110000 UTC+0.0 s
2009-11-120000 UTC+0.1 s
2009-06-110000 UTC+0.2 s
2009-03-120000 UTC+0.3 s
2008-11-200000 UTC- 0.6 s
2008-08-070000 UTC- 0.5 s
2008-03-130000 UTC- 0.4 s
2007-11-290000 UTC- 0.3 s
2007-06-140000 UTC- 0.2 s
2007-03-150000 UTC- 0.1 s
2006-12-220000 UTC+0.0 s
2006-09-280000 UTC+0.1 s
2006-04-270000 UTC+0.2 s
2006-01-010000 UTC+0.3 s
2005-03-170000 UTC- 0.6 s
2004-04-290000 UTC- 0.5 s
2003-04-030000 UTC- 0.4 s
2002-10-240000 UTC- 0.3 s
2002-02-140000 UTC- 0.2 s
2001-10-040000 UTC- 0.1 s
2001-03-010000 UTC+0.0 s