Leap Seconds

Leap seconds are added to keep the clocks synchronized with the Earth's rotation.

Basic details

The second is the base unit for modern time keeping. The second was previously defined based on the Earth's rotation, but because modern atomic clocks are more accurate than the Earth's rotation the definition was changed in 1967. A second is currently defined as being the duration of 9,192,631,770 periods/oscillations of radiation from a Cesium-133 atom at the ground state (near 0 Kelvin - coldest possible).

The Earth is rotating slower and slower over time, while the atomic clocks are not slowing down. On one average day the difference is around 0.002 seconds, which means around 1 second in 500 days. In order to synchronize the atomic clocks with the Earth's observed rotation, the atomic clocks are occasionally instructed to add an extra second – the leap second. Leap seconds are inserted so that the difference between the UTC (Coordinated Universal Time) and UT1 (mean solar time - observed Earth rotation) is kept below 0.9 seconds.

The leap second is added in the end of June or December. It is also possible to have a negative leap second, where one second is removed, in a case where the Earth is rotating faster, but such a negative second has never been used, and is rather unlikely to be used in the future.

How are leap seconds declared?

The International Earth Rotation and Reference System Service (IERS) observes the Earth's rotation and nearly 6 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.

How leap seconds are 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.

Leap second on 2005-12-31 23:59:60 UTC

The last leap second was inserted like this, in the UTC time scale, and corresponding times elsewhere in the world. (2005-12-31 means December 31, 2005, and 2006-01-01 means January 1, 2006).

UTC Date	UTC Time	Local time world-wide
2005-12-31	23:59:57	Corresponding times
2005-12-31	23:59:58	Corresponding times
2005-12-31	23:59:59	Corresponding times
2005-12-31	23:59:60	Leap second added
2006-01-01	00:00:00	Corresponding times
2006-01-01	00:00:01	Corresponding times
2006-01-01	00:00:02	Corresponding times

No leap seconds in June 2008

The latest leap second bulletin sent by IERS on 18 January 2008 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 26
FAX : 33 (0) 1 40 51 22 91
Internet : services.iers@obspm.fr

 Paris, 18 January 2008
 

 Bulletin C 35
 
 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 2008.
 The difference between Coordinated Universal Time UTC and the 
 International Atomic Time TAI is :		
		
 from 2006 January 1, 0h UTC, until further notice : UTC-TAI = -33 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

The next message will be announced in June 2008 and will tell if there will be a leap second in December 2008 (earliest possiblity).

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

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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