Every leap second since 1972, with dates and offsets
Key facts
- 27 leap seconds have been inserted, the first effective on 1 July 1972 and the most recent on 1 January 2017. [1]Official
- The IERS file holds 28 rows. The first, 1 January 1972 with TAI minus UTC of 10 s, is the initial value and not an insertion. [1]Official
- The count follows from the file itself: 37 s minus 10 s is 27 inserted seconds. [1]Official
- Every insertion took effect at the start of 1 January or 1 July, and none has been negative. [1]Official
- The edition used here is updated through IERS Bulletin 72 of July 2026 and expires on 28 June 2027. [1]Official
This is the full list of leap seconds, taken from the data file published by the IERS Earth Orientation Centre [1]Official.
27 seconds have been inserted. The first took effect on 1 July 1972, the most recent on 1 January 2017, and none has been inserted since [1]Official.
How to read the file, and why it has 28 rows
The IERS file holds 28 rows, but only 27 of them are insertions [1]Official.
The first row, 1 January 1972 with TAI − UTC of 10 s, is the starting point. It records the offset that UTC began with in its present form, not a second that was added on that day [1]Official.
Every later row raises TAI − UTC by exactly one second. That is why the count of insertions is the difference between the last value and the first: 37 s − 10 s = 27 [1]Official.
The file header states the same reading in its own words: each value of TAI − UTC is valid from its own epoch until the epoch on the next line, and the last line means that no leap second has been introduced since that date [1]Official.
What each column means
- Effective date. The moment the new offset starts. The second itself is inserted at the end of the previous day, so the value applies from 00:00:00 UTC on the date shown [1]Official.
- TAI − UTC after. The whole number of seconds by which atomic time leads UTC from that date onwards [1]Official.
- MJD. The Modified Julian Date of the same moment, which is the form the file itself uses [1]Official.
The complete list
The first row is the initial value of 1972, marked as such. The 27 rows below it are the insertions [1]Official.
| Effective date | TAI − UTC after | MJD | Source |
|---|---|---|---|
| 1 January 1972 | 10 s — initial value, not an insertion | 41317.0 | [1]Official |
| 1 July 1972 | 11 s | 41499.0 | [1]Official |
| 1 January 1973 | 12 s | 41683.0 | [1]Official |
| 1 January 1974 | 13 s | 42048.0 | [1]Official |
| 1 January 1975 | 14 s | 42413.0 | [1]Official |
| 1 January 1976 | 15 s | 42778.0 | [1]Official |
| 1 January 1977 | 16 s | 43144.0 | [1]Official |
| 1 January 1978 | 17 s | 43509.0 | [1]Official |
| 1 January 1979 | 18 s | 43874.0 | [1]Official |
| 1 January 1980 | 19 s | 44239.0 | [1]Official |
| 1 July 1981 | 20 s | 44786.0 | [1]Official |
| 1 July 1982 | 21 s | 45151.0 | [1]Official |
| 1 July 1983 | 22 s | 45516.0 | [1]Official |
| 1 July 1985 | 23 s | 46247.0 | [1]Official |
| 1 January 1988 | 24 s | 47161.0 | [1]Official |
| 1 January 1990 | 25 s | 47892.0 | [1]Official |
| 1 January 1991 | 26 s | 48257.0 | [1]Official |
| 1 July 1992 | 27 s | 48804.0 | [1]Official |
| 1 July 1993 | 28 s | 49169.0 | [1]Official |
| 1 July 1994 | 29 s | 49534.0 | [1]Official |
| 1 January 1996 | 30 s | 50083.0 | [1]Official |
| 1 July 1997 | 31 s | 50630.0 | [1]Official |
| 1 January 1999 | 32 s | 51179.0 | [1]Official |
| 1 January 2006 | 33 s | 53736.0 | [1]Official |
| 1 January 2009 | 34 s | 54832.0 | [1]Official |
| 1 July 2012 | 35 s | 56109.0 | [1]Official |
| 1 July 2015 | 36 s | 57204.0 | [1]Official |
| 1 January 2017 | 37 s | 57754.0 | [1]Official |
What the list shows
Insertions were frequent at the start and have become rare. Nine seconds were inserted between 1972 and 1980, and five between 2006 and 2017 [1]Official.
There are also long gaps. No second was inserted between 1 January 1999 and 1 January 2006, and none since 1 January 2017 [1]Official.
The pattern is not a rule. Each insertion answers a measurement of the Earth’s rotation, which is why no list of future dates exists: when is the next leap second sets out what has been announced, which is nothing.
Why the list may stop growing
Resolution 4 of the 27th CGPM, in 2022, decides that the maximum value for the difference UT1 − UTC will be increased in, or before, 2035 [2]Official. A larger limit means insertions become rarer still.
Decision CIPM/115-5 of 2026 endorses an updated Draft Resolution C that fixes 20 May 2027 as the implementation date of the new maximum value [3]Official. That text is a proposal for the 28th meeting of the CGPM on 13–15 October 2026, and no vote has been taken [4]Official.
Until then this table stays as it is, and the rules behind it stay as they are.
Using these numbers in code
Read the offsets from the IERS file rather than from memory. The published file is the reference, and its current edition expires on 28 June 2027, after which a new edition carries the same series forward [1]Official.
Our own lunar figures are not affected by any of this: every lunar value on this site is a model value that ignores leap seconds entirely, as the methodology page states [5].
What a leap second is, and who announces one, is explained in what is a leap second. The five steps that produce UTC are set out in how UTC is calculated, and the Learn hub lists our other articles.
Sources
Last verified