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Leap second and UT1-UTC information

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).

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

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 2019.

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

Date MJD Date MJD Date MJD Date MJD
2016-12-31 57753 1998-12-31 51178 1989-12-31 47891 1979-12-31 44238
2015-06-30 57203 1997-06-30 50629 1987-12-31 47160 1978-12-31 43873
2012-06-30 56108 1995-12-31 50082 1985-06-30 46246 1977-12-31 43508
2008-12-31 54831 1994-06-30 49533 1983-06-30 45515 1976-12-31 43143
2005-12-31 53735 1993-06-30 49168 1982-06-30 45150 1975-12-31 42777
    1992-06-30 48803 1981-06-30 44785 1974-12-31 42412
    1990-12-31 48256     1973-12-31 42047
            1972-12-31 41682
            1972-06-30 41498

 

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

Date MJD UT1-UTC(±5 ms) UTC(USNO,MC) - UTC(NIST) (±20 ns)
2020-08-27 59088 -191.20 ms -1.9 ns
2020-08-20 59081 -193.67 ms -2.2 ns
2020-08-13 59074 -200.38 ms -0.8 ns
2020-08-06 59067 -203.89 ms -1.2 ns
2020-07-30 59060 -210.18 ms -2.1 ns
2020-07-23 59053 -214.69 ms -1.4 ns
2020-07-16 59046 -223.76 ms -2.4 ns
2020-07-09 59039 -231.29 ms -2.0 ns
2020-07-02 59032 -239.49 ms -1.8 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

Start Date Time DUT1 Correction
2019-05-02 0000 UTC -0.2 s
2019-01-17 0000 UTC -0.1 s
2018-09-21 0000 UTC +0.0 s
2018-03-15 0000 UTC +0.1 s
2017-11-30 0000 UTC +0.2 s
2017-06-29 0000 UTC +0.3 s
2017-03-30 0000 UTC +0.4 s
2017-01-26 0000 UTC +0.5 s
2017-01-01 0000 UTC +0.6 s
2016-11-17 0000 UTC -0.4 s
2016-09-01 0000 UTC -0.3 s
2016-05-19 0000 UTC -0.2 s
2016-03-24 0000 UTC -0.1 s
2016-01-31 0000 UTC  0.0 s
2015-11-26 0000 UTC +0.1 s
2015-09-11 0000 UTC +0.2 s
2015-07-01 0000 UTC +0.3 s
2015-05-28 0000 UTC - 0.7 s
2015-03-19 0000 UTC - 0.6 s
2014-12-25 0000 UTC - 0.5 s
2014-09-25 0000 UTC - 0.4 s
2014-05-08 0000 UTC - 0.3 s
2014-02-20 0000 UTC - 0.2 s
2013-11-21 0000 UTC - 0.1 s
2013-08-22 0000 UTC + 0.0 s
2013-04-11 0000 UTC + 0.1 s
2013-01-31 0000 UTC + 0.2 s
2012-10-25 0000 UTC + 0.3 s
2012-07-01 0000 UTC + 0.4 s
2012-05-10 0000 UTC - 0.6 s
2012-02-09 0000 UTC - 0.5 s
2011-11-04 0000 UTC - 0.4 s
2011-05-12 0000 UTC - 0.3 s
2011-01-06 0000 UTC - 0.2 s
2010-06-03 0000 UTC - 0.1 s
2010-03-11 0000 UTC +0.0 s
2009-11-12 0000 UTC +0.1 s
2009-06-11 0000 UTC +0.2 s
2009-03-12 0000 UTC +0.3 s
2008-11-20 0000 UTC - 0.6 s
2008-08-07 0000 UTC - 0.5 s
2008-03-13 0000 UTC - 0.4 s
2007-11-29 0000 UTC - 0.3 s
2007-06-14 0000 UTC - 0.2 s
2007-03-15 0000 UTC - 0.1 s
2006-12-22 0000 UTC +0.0 s
2006-09-28 0000 UTC +0.1 s
2006-04-27 0000 UTC +0.2 s
2006-01-01 0000 UTC +0.3 s
2005-03-17 0000 UTC - 0.6 s
2004-04-29 0000 UTC - 0.5 s
2003-04-03 0000 UTC - 0.4 s
2002-10-24 0000 UTC - 0.3 s
2002-02-14 0000 UTC - 0.2 s
2001-10-04 0000 UTC - 0.1 s
2001-03-01 0000 UTC +0.0 s
Created August 6, 2009, Updated October 14, 2020