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NIST Time Scale Data Archive
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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: No leap second will be added at the end of December 2009.Leap Seconds Inserted into the UTC Time Scale
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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) |
2010-02-25 | 55252 | 061 ms |
2010-02-18 | 55245 | 068 ms | 2010-02-11 | 55238 | 076 ms |
2010-01-04 | 555231 | 082 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
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
Online: January 1998 - Last update: February 25, 2010 - ep