Leap second

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Leap second as it would be displayed on a digital watch set to the UTC time zone.

The earth rotates slightly more slowly than was taken as the basis for the definition of the second ; an actual mean sunny day therefore lasts a fraction of a second longer than 86400 seconds. This effect accumulates. Therefore, from time to time a leap second to the Coordinated Universal Time (UTC), an atomic time scale, added to them with that on the rotation of the earth -based world time (UT1), the approximately with the previously used middle Greenwich solar time matches (GMT), possible to be kept synchronous (the amount of the difference DUT1 = UT1 - UTC should be less than 0.9 s).

In a way, leap seconds are comparable to leap days :

  • Leap day : Since the earth needs a little longer than exactly 365 days to orbit the sun until it is back at the same point of its orbit - around the spring equinox - (the mean orbital speed of the earth is "too slow"), it becomes a leap day inserted before the deviation of the calendar from the actual astronomical situation exceeds a whole day.
  • Leap Second : As the earth in its rotation around itself in the middle a little longer than just 24 hours, so 86,400 seconds, takes up the sun on its apparent path back to their peak reached (the day is "too long"), a leap second inserted before the deviation of the coordinated universal time (which is based on the definition of the second) from the mean solar day (which is based on the definition of the astronomical situation) exceeds a whole second.

Since the rotation speed of the earth has non-periodic irregularities that cannot be calculated in advance, leap seconds are inserted as required and not according to a fixed pattern. At the moment this is necessary on average about every 18 months.

Leap seconds are set by the International Service for Earth Rotation and Reference Systems. In most cases , however, a state institution is responsible for the legal time in a particular country.


Until the 1950s, the second was defined as 1/86400 of a mean sunny day . However , a time scale derived from the position of the sun and thus ultimately from the rotation of the earth does not run strictly uniformly, since the rotational speed of the earth is subject to irregular fluctuations and a long-term slowdown. Such a non-uniform time scale is not useful for many technical and scientific purposes.

Since the subsequent ephemeris second did not meet the requirements either, the second has been defined by a precisely determined radiation frequency since 1967, namely as 9,192,631,770 times the period of the radiation corresponding to the transition between the two hyperfine levels of the ground state of atoms of the nuclide 133 Cs . The uniform time scale generated in this way is International Atomic Time (TAI). The earth's rotation is still observed; the unevenly running time scale derived from it is the Universal Time UT1. B. is required for astronomical purposes.

In 1972 the difference between UTC and TAI was 10 seconds before the leap second was introduced; in the first half of 2020 it will be 37 seconds.

Although the second has been redefined several times over time, its length has been kept as good as possible with each change of definition. Therefore, the length of today's SI second is ultimately based on determinations of the length of the mean solar day from the late 19th century. The variability of the mean solar day was not yet known at the time, and its mean length had been determined from observational material that spanned mainly the 18th and 19th centuries. As a result, today's second is representative of the length of the astronomically determined second around the middle of the observation period evaluated at that time, i.e. around 1820. Currently, the solar day is a few milliseconds longer than it was then, so that the UT1 second derived from the Earth's rotation is also slightly longer than the SI second, which reflects the situation 200 years ago. To compensate, (positive) leap seconds are inserted in UTC. Should the earth's own rotation accelerate and the mean sunny day be shorter than 86400 SI seconds, negative leap seconds can also be "inserted", ie individual seconds can be left out. So far, all leap seconds have been positive.

1962 to 2016: Deviation in day lengths (−1.1 ... + 4.4 ms) and cumulative shift (−7 ... + 27 s) based on 1972


List of all leap seconds after 23:59:59 UTC
1972: TAI = UTC + 10 s
year June 30th 31 Dec Δ
1972 + 1 s + 1 s +12 s
1973 - + 1 s +13 s
1974 - + 1 s +14 s
1975 - + 1 s +15 s
1976 - + 1 s +16 s
1977 - + 1 s +17 s
1978 - + 1 s +18 s
1979 - + 1 s +19 s
1980 - - +19 s
1981 + 1 s - +20 s
1982 + 1 s - +21 s
1983 + 1 s - +22 s
1984 - - +22 s
1985 + 1 s - +23 s
1986 - - +23 s
1987 - + 1 s +24 s
1988 - - +24 s
1989 - + 1 s +25 s
1990 - + 1 s +26 s
1991 - - +26 s
1992 + 1 s - +27 s
1993 + 1 s - +28 s
1994 + 1 s - +29 s
1995 - + 1 s +30 s
1996 - - +30 s
1997 + 1 s - +31 s
1998 - + 1 s +32 s
1999 - - +32 s
2000 - - +32 s
2001 - - +32 s
2002 - - +32 s
2003 - - +32 s
2004 - - +32 s
2005 - + 1 s +33 s
2006 - - +33 s
2007 - - +33 s
2008 - + 1 s +34 s
2009 - - +34 s
2010 - - +34 s
2011 - - +34 s
2012 + 1 s - +35 s
2013 - - +35 s
2014 - - +35 s
2015 + 1 s - +36 s
2016 - + 1 s +37 s
2017 - - +37 s
2018 - - +37 s
2019 - - +37 s
2020 - - +37 s
Σ + 11 s + 16 s
+ 27 s
Total: TAI = UTC + 37 s

The earth's rotation is subject to constant fluctuations due to a large number of influences. It has accelerated in the last few decades, but in the long term it will be slowed down by the tidal friction . This is why UT1 runs slower and slower in the long term, while TAI runs strictly uniformly.

The official time used in everyday life should expediently be based on the day-night change, i.e. on the earth's rotation and thus on UT1; on the other hand, a strictly uniform time measure, i.e. TAI, is desirable for technical purposes. Coordinated Universal Time UTC was introduced as a compromise . As for TAI, the time unit for UTC is the SI second measured with atomic clocks . By inserting leap seconds, UTC never moves more than 0.9 seconds away from UT1. This ensures, on the one hand, that the widespread UTC does not drift away from the mean solar time in the long term, and on the other hand, a time unit accurate to the atomic clock is available.

Practical implementation

Leap seconds are set in Germany by the Physikalisch-Technische Bundesanstalt , which adheres to the leap seconds set internationally by the International Service for Earth Rotation and Reference Systems. On average, a leap second is necessary about every 18 months and is given priority on December 31 or June 30, subordinated on March 31 or September 30 after 23:59:59 UTC (i.e. before 01:00  CET or 02:00  CEST ) inserted. Since the introduction of the system in 1972, only the times in December and June have been used.

The instruction to insert a leap second is given whenever it is to be expected in the near future that the difference between UTC and UT1 will increase over 0.9 seconds. After 23:59:59 UTC on the specified days, an additional second is inserted at 23:59:60 before the clock advances to 00:00:00 on the following day. This means that the day with the leap second consists of 86,401 atomic seconds instead of the usual 86,400. In the event that the earth's rotation would become significantly faster, negative leap seconds are also provided. In this case, the next day at 00:00:00 would immediately follow 23:59:58. However, this has never happened before. The earth's rotation has accelerated slightly since the end of the 1970s (against the background of the usual fluctuations), so that leap seconds have not been necessary since then. However, a day is still longer than the nominal 86,400 atomic seconds, so that it is likely that positive leap seconds will still have to be compensated for at more or less regular intervals.

However, the correct representation of 23:59:60 is rarely seen on commercially available watches. All clocks that are only clocked by their own timer (usually a quartz crystal) have no information about when a leap second should be taken into account. Therefore, in principle, only such clocks can correctly display a leap second that are transmitted in other ways, e.g. B. time signal or NTP , receive information about an upcoming leap second. The time signal DCF77 contains the information whether a (positive) leap second is to be inserted at the end of the current hour. If a radio clock receives the time signal during this period, it could theoretically be displayed correctly. However, with radio-controlled clocks it is quite common to receive the time signal much less frequently (about once a day), so that the leap second is usually not received here either. In this case, the leap second is initially not taken into account, so that the clock advances by one second between 23:59:59 and the next time signal reception.


Due to the irregularity of the slowing down of the earth's rotation, a prediction of whether and when a leap second will be necessary is only possible for the near future. It is the job of the International Service for Earth's Rotation and Reference Systems (IERS) to observe the Earth's rotation and determine whether a leap second is necessary. Your determination will be published in the semi-annual Bulletin  C six months before the date. The previous leap second was added on December 31, 2016 (UTC ± 0).

Since this process can be a source of errors for computer systems, consideration is given to whether the leap seconds should be inserted less frequently or whether the system should be fundamentally changed. If, for example, a whole switching hour were inserted in the year 2600, the frequent small adjustments, which have to be made at irregular intervals, could be dispensed with. The decision was initially postponed in 2012 to 2015 at the earliest. In the course of the ITU World Radiocommunications Conference 2015 (WRC-15) it was decided that further investigations are necessary for a decision on the abolition of leap seconds. At least until the World Radiocommunication Conference in 2023, the previous regulation on the insertion of leap seconds as set out in ITU-R TF.4060-6 will remain in place.

The Global Positioning System (GPS) has been using an atomic time scale without leap seconds since its introduction in 1980, so as not to lose the rhythm. The GPS time is therefore 18 seconds ahead of UTC (as of January 2020). The current difference is also transmitted in the GPS signal in order to make the UTC available to the second for GPS receivers and users.


The longest year in the Gregorian calendar to date in the time zones UTC ± 0 and west was 1972. As a leap year, it was one day and two leap seconds longer than usual. The shortest year was 1582 when the introduction of the Gregorian calendar skipped the ten days between October 4th and 15th.

The leap second on June 30, 2012 caused significant problems on many servers around the world. Especially computers with certain Linux kernel versions were affected. By a bug ( bugs ) in the kernel, there was a so-called deadlock . Java -based programs and MySQL databases were mainly affected . In addition to the airline Qantas , which had to postpone around 50 flights, many large websites were affected, which has already led trade magazines to compare them with the Millennium Bug . The company Google got around the problem with a trick in which small time increments for the time control of the NTP server were added up over the whole day until the time difference was equalized.

See also


Web links

Wiktionary: leap second  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Christian Speicher: Time change: The next leap second is due In: Neue Zürcher Zeitung of December 30, 2016.
  2. Realization of the SI second . Working group 4.41 of the PTB. August 14, 2013. Retrieved September 16, 2013.
  3. TAI − UTC (Jan. 1, 1972 - Jun. 28, 2021) , IERS , July 7, 2020.
  4. Bulletin C 60 , IERS , July 7, 2020.
  5. http://support.casio.com/en/manual/009/qw5110.pdf
  6. ^ Leap second announcements in UTC . International Earth Rotation and Reference Systems Service (IERS); accessed on December 30, 2018.
  7. Christian Speicher: Is the leap second a problem? For years there have been efforts to abolish leap seconds. Now is to be voted on at the highest level. NZZ , January 11, 2012; accessed on December 4, 2019.
  8. Kenneth Chang: Decision About One Second Is Postponed for Three Years . The New York Times , Jan. 19, 2012; Retrieved September 5, 2014.
  9. Coordinated Universal Time (UTC) to retain “leap second” . International Telecommunication Union (ITU), Press Release November 2015; Retrieved April 28, 2016.
  10. Ferdinand Thommes: Leap second on the weekend paralyzed many computers. PC Magazin, July 3, 2012, accessed July 4, 2012 .