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About Leap Seconds

A leap second is a second, as measured by an atomic clock, added to or subtracted from Coordinated Universal Time (UTC) to make it agree with astronomical time to within 0.9 second. It compensates for slowing in the Earth’s rotation and is added during the end of June or December. It is important to look at how seconds are used in relation to modern time keeping to gain an understanding of the concept of the leap second and why it is used.

No positive leap second will be introduced at the end of June 2010.

How Leap Seconds Declared?

The International Earth Rotation and Reference System Service (IERS) observes the Earth's rotation and nearly six months in advance (January and July) a "Bulletin C" message is sent out, which reports whether or not to add a leap second in the end of June and December.

IERS schedules a leap second as needed to keep the time difference between atomic clocks and Earth’s rotation to below 0.9 seconds.

How are Leap Seconds Inserted?

Leap seconds are inserted at the end of June or December as an additional second after 23:59:59 UTC (Universal Time Coordinated). The additional second is the 61st second of the last minute of the month, and it is written as 23:59:60 (or 11:59:60 PM in 12-hour format).

The second is inserted at the same time all over the world - the actual local time will therefore depend on the time zone. Only regions in the UTC time zone will add the second just before midnight, for time zones east of UTC, the second will be added the next day (first day in January or July), for time zones west of UTC, the second will be added earlier on the same day as for UTC.

Last leap second on 2008-12-31 23:59:60 UTC

The last leap second was inserted like this, in the UTC time scale, and corresponding times elsewhere in the world. (2008-12-31 means December 31, 2008, and 2009-01-01 means January 1, 2009).

UTC DateUTC TimeLocal time world-wide
2008-12-3123:59:57Corresponding times
2008-12-3123:59:58Corresponding times
2008-12-3123:59:59Corresponding times
2008-12-3123:59:60Leap second added
2009-01-0100:00:00Corresponding times
2009-01-0100:00:01Corresponding times
2009-01-0100:00:02Corresponding times

IERS Announcement of January 2010

A leap second bulletin sent by IERS on January 14, 2010 is displayed here:

       INTERNATIONAL EARTH ROTATION AND REFERENCE SYSTEMS SERVICE (IERS) 

SERVICE INTERNATIONAL DE LA ROTATION TERRESTRE ET DES SYSTEMES DE REFERENCE

SERVICE DE LA ROTATION TERRESTRE
OBSERVATOIRE DE PARIS                                   
61, Av. de l'Observatoire 75014 PARIS (France)
Tel.      : 33 (0) 1 40 51 22 29
FAX       : 33 (0) 1 40 51 22 91
Internet  : services.iers@obspm.fr

                                               Paris, 14 January 2010
                                                           

                                               Bulletin C 39
                                
                                               To authorities responsible                   
                                               for the measurement and 
                                               distribution of time             
                                                                                        


                          INFORMATION ON UTC - TAI
                      

 NO positive leap second will be introduced at the end of June 2010.
 The difference between Coordinated Universal Time UTC and the 
 International Atomic Time TAI is :		
		
     from 2009 January 1, 0h UTC, until further notice : UTC-TAI = -34 s

 Leap seconds can be introduced in UTC at the end of the months of December 
 or June,  depending on the evolution of UT1-TAI. Bulletin C is mailed every  
 six months, either to announce a time step in UTC, or to confirm that there 
 will be no time step at the next possible date.


                                           Daniel GAMBIS
                                           Head			
                                           Earth Orientation Center of the IERS
                                           Observatoire de Paris, France



Historic leap seconds

The following table shows all leap seconds that have been added so far.

UTC–TAI means the difference between the civil time (UTC) which is kept within 0.9 seconds from Earth's rotation and the International Atomic Time (TAI) which does not care about the Earth's rotation, but rather observations of the Cesium-133 atom. A difference of 33 seconds means that the Earth has slowed by 33 seconds compared with TAI since 1958 (when TAI and UTC were the same). The difference between UTC and TAI was defined as 10 seconds from January 1972 and the first leap second was added in June 1972.

UTC DateUTC TimeUTC–TAI after insertion
1972-06-3023:59:60-11 seconds
1972-12-3123:59:60-12 seconds
1973-12-3123:59:60-13 seconds
1974-12-3123:59:60-14 seconds
1975-12-3123:59:60-15 seconds
1976-12-3123:59:60-16 seconds
1977-12-3123:59:60-17 seconds
1978-12-3123:59:60-18 seconds
1979-12-3123:59:60-19 seconds
1981-06-3023:59:60-20 seconds
1982-06-3023:59:60-21 seconds
1983-06-3023:59:60-22 seconds
1985-06-3023:59:60-23 seconds
1987-12-3123:59:60-24 seconds
1989-12-3123:59:60-25 seconds
1990-12-3123:59:60-26 seconds
1992-06-3023:59:60-27 seconds
1993-06-3023:59:60-28 seconds
1994-06-3023:59:60-29 seconds
1995-12-3123:59:60-30 seconds
1997-06-3023:59:60-31 seconds
1998-12-3123:59:60-32 seconds
2005-12-3123:59:60-33 seconds
2008-12-3123:59:60-34 seconds
Next leap second not yet known, earliest chance is 2010-12-31

Future for leap seconds

There have been proposals for changing the current time scale, so that UTC is no longer tied so closely with the earth's rotation. Over years, this will lead to minutes and eventually hours of difference, so maybe something like a leap hour will be needed to maintain some synchronization between the day and night and the clock.

It is not yet decided what will happen.

Leap second support on timeanddate.com

Currently leap seconds are not supported on timeanddate.com, but such support is likely to be added in the future.
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