Why clocks need a 61-second minute
At the end of a UTC day, on 30 June or 31 December, the clock can read 23:59:60. That minute runs 61 seconds. The extra beat keeps official time within 0.9 seconds of the Sun.
Atomic clocks do not drift. Earth does. Tidal friction from the Moon drags on the planet’s rotation, adding roughly 1.7 milliseconds to the day each century. Small, but relentless. Without the correction, noon would wander away from the Sun. The leap second yanks it back.
Earth’s rotation is not a clock
The Moon pulls on the oceans. That pull brakes the spin. Each century, the day stretches. Over decades, the gap between atomic time and solar time widens enough to matter.
A leap second is not a sign something broke. It is a sign the planet is alive.
How many leap seconds have been added since 1972?
The first leap second landed on 30 June 1972. Every one since has been positive: a second added, never removed. The rhythm is irregular. Some years got none. 1997 got two.
No leap second has been inserted since the end of 2016.
| Date | Leap second |
|---|---|
| 30 June 1972 | +1 |
| 31 December 1972 | +1 |
| 31 December 1973 | +1 |
| 31 December 1974 | +1 |
| 31 December 1975 | +1 |
| 31 December 1976 | +1 |
| 31 December 1977 | +1 |
| 31 December 1978 | +1 |
| 31 December 1979 | +1 |
| 30 June 1981 | +1 |
| 30 June 1982 | +1 |
| 30 June 1983 | +1 |
| 30 June 1985 | +1 |
| 31 December 1987 | +1 |
| 31 December 1989 | +1 |
| 31 December 1990 | +1 |
| 30 June 1992 | +1 |
| 30 June 1993 | +1 |
| 30 June 1994 | +1 |
| 31 December 1995 | +1 |
| 30 June 1997 | +1 |
| 31 December 1998 | +1 |
| 31 December 2005 | +1 |
| 31 December 2008 | +1 |
| 30 June 2012 | +1 |
| 30 June 2015 | +1 |
| 31 December 2016 | +1 |
When was the last leap second?
At midnight UTC on the final day of 2016, clocks displayed 23:59:60 before turning to 1 January 2017. The day ran one second long. Most people missed it. Computers did not.
The mid-2012 insertion knocked Reddit, LinkedIn and other services offline. The end-of-2016 event was smoother, but the pattern was clear: adding seconds breaks software. That insertion was the 27th. It remains the last.
Why are leap seconds being abolished?
In November 2022, the General Conference on Weights and Measures (CGPM) resolved to abolish leap seconds by 2035. The vote was not unanimous. Technology’s pain outweighed astronomy’s preference.
After 2035, UTC will stop adjusting. The gap between atomic time and solar time will be allowed to grow. For daily life, the drift is invisible. For astronomers and satellite operators, it is a permanent shift they will need to track.
What a leap second does to a duration calculation
A day with a leap second contains 86,400 seconds plus one. Most consumer tools ignore the extra tick. They treat every day as a flat 86,400 seconds. For timesheets, that is fine. For satellite tracking or financial timestamps, it is not.
If you calculate a duration that crosses a leap second boundary, the math changes by exactly one second. Precision tools must account for it. Casual tools skip it.
What clock never pauses for leap seconds?
International Atomic Time (TAI) is the continuous scale. It never inserts a second. It never skips one. UTC is TAI minus the accumulated leap seconds.
Since 1972, that offset has grown. As of 2026, UTC = TAI − 27 seconds. The SI second, defined since 1967 by the cesium atom’s 9,192,631,770 cycles, underpins both. TAI counts those cycles without interruption. UTC stops, corrects, and carries on.