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 the 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.
A positive leap second will be introduced at the end of June 2012.
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.
Next leap second on 2012-06-30 23:59:60 UTC
The last leap second was inserted like this, in the UTC time scale, and corresponding times elsewhere in the world. (2012-06-30 means June 30, 2012, and 2012-07-01 means July 1, 2012).
| UTC Date | UTC Time | Local time world-wide |
|---|---|---|
| 2012-06-30 | 23:59:57 | Corresponding times |
| 2012-06-30 | 23:59:58 | Corresponding times |
| 2012-06-30 | 23:59:59 | Corresponding times |
| 2012-06-30 | 23:59:60 | Leap second added |
| 2012-07-01 | 00:00:00 | Corresponding times |
| 2012-07-01 | 00:00:01 | Corresponding times |
| 2012-07-01 | 00:00:02 | Corresponding times |
IERS Announcement of January 2012
A leap second bulletin sent by IERS on January 5, 2012 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
e-mail : services.iers@obspm.fr
http://hpiers.obspm.fr/eop-pc
Paris, 5 January 2012
Bulletin C 43
To authorities responsible
for the measurement and
distribution of time
UTC TIME STEP
on the 1st of July 2012
A positive leap second will be introduced at the end of June 2012.
The sequence of dates of the UTC second markers will be:
2012 June 30, 23h 59m 59s
2012 June 30, 23h 59m 60s
2012 July 1, 0h 0m 0s
The difference between UTC and the International Atomic Time TAI is:
from 2009 January 1, 0h UTC, to 2012 July 1 0h UTC : UTC-TAI = - 34s
from 2012 July 1, 0h UTC, until further notice : UTC-TAI = - 35s
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 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 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 |
| 2008-12-31 | 23:59:60 | -34 seconds |
| 2012-06-30 | 23:59:60 | -35 seconds |
| Next leap second not yet known, earliest date is 2012-12-31 | ||
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.More information
- The Leap Second Explained
- The Future of Leap Seconds
- Why Leap Years are Used
- Time Zone News
- List of countries that observe Daylight Saving Time in 2012
- Time Zone Abbreviations
Related links
- The World Clock – Current time all over the world
- Personal World Clock
- Meeting Planner
- Time Zone Converter
- Event Time Announcer/Fixed Time – Show local times worldwide for your event.