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Leap second and UT1-UTC information

This page 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, GOES, ACTS, and Network Time Service (NTS) 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 also information about why we need leap seconds.

Leap seconds

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

NOTE: A positive leap second will be added at the end of December 2008.

Leap Seconds Inserted into the UTC Time Scale

Date MJD 2008-12-31 54831 2005-12-31 53735

Date MJD 1998-12-31 51178 1997-06-30 50629 1995-12-31 50082 1994-06-30 49533 1993-06-30 49168 1992-06-30 48803 1990-12-31 48256

Date MJD 1989-12-31 47891 1987-12-31 47160 1985-06-30 46246 1983-06-30 45515 1982-06-30 45150 1981-06-30 44785

Date MJD 1979-12-31 44238 1978-12-31 43873 1977-12-31 43508 1976-12-31 43143 1975-12-31 42777 1974-12-31 42412 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)
2009-01-29	54860	385 ms
2009-01-22	54853	387 ms
2009-01-15	54846	392 ms
2009-01-08	54839	399 ms
2009-01-01	54832	407 ms

Here is an archive of daily UT1-UTC values.

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, GOES, 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
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