Timekeeping · IERS 1962–2026
Twenty-Seven Seconds, Then Silence
Twenty-seven times between 1972 and 2016, the world's atomic clock gained an extra second so it wouldn't drift from Earth's actual spin. Since the last one, on 31 December 2016, none has been needed: the planet has entered a decade in which it turns, on average, faster than any other on direct record. In October, in Versailles, the world's timekeepers vote on retiring the mechanism before it can demand, for the first time ever, a second taken away.
The mechanism
A dial reset twenty-seven times
The civil second has a fixed definition: 9,192,631,770 oscillations of a caesium-133 atom, whatever the planet's spin happens to be doing that day. Earth's rotation, by contrast, changes from one day to the next, pushed by tidal friction, currents in the liquid core and, in recent years, melting ice. The gap between astronomical time and atomic time is called UT1−UTC, and a Paris institute tracks it and publishes the figure every month. The dial below shows exactly that gap, month by month, from 1962 to today: move the slider and watch the needle.
July 2026
27 of 27 leap seconds used so far.
The international treaty governing universal time requires the gap to never exceed 0.9 seconds. Each time the needle approached that threshold, the Paris institute announced a leap second: at midnight between 30 June and 1 July, or between 31 December and 1 January, the world's clocks counted 23:59:60 before rolling over. It happened 27 times, always to pull the needle back toward zero. Since 2016, the needle hasn't needed a single correction.
The anomaly
The shortest day ever measured
The explanation sits in the same Paris record. Since 1962, the shortest day ever registered by an atomic clock was 5 July 2024: 1.65 milliseconds shorter than the standard 86,400 seconds. Before 2020, the record had stood at just 1.05 milliseconds. Since then, Earth has broken that record nearly every summer, driven mostly by the Moon's position relative to the equator.
Viewed by decade, the six complete decades in the official record tell the same story, in reverse: the day ran longer than the 86,400-second reference, from 2.84 milliseconds in the 1970s down to 0.61 milliseconds in the 2000s. The current decade, measured through mid-2026, is the first of the 64 years on record with a negative average.
* the 2020s, calculated over the span elapsed through July 2026, not a full decade. Milliseconds per day, against the standard 86,400 seconds.
The paradox is that this recent braking of the acceleration has a climate cause of its own. Geophysicist Duncan Agnew calculated, in a study published in Nature in 2024, that without the melting of the Greenland and Antarctic ice sheets, a negative leap second would already have been needed, around 2026. The meltwater, redistributed toward the equator, slows the planet's spin just enough to postpone the reckoning by about three years, to around 2029.
The vote
The spare clock
Nobody wants to find out what a negative leap second does. The few incidents caused by positive ones, such as the 2012 leap second that knocked out Reddit and other internet services for hours, or the 31 December 2016 leap second that triggered errors in Cloudflare's domain-name system, show how fragile software can be around a clock that, in theory, can also run backward. Almost no software has ever been tested for the opposite case: subtracting a second, not adding one.
NO LEAP SECOND WILL BE INTRODUCED AT THE END OF DECEMBER 2026.
UTC−TAI = −37 s SINCE 1 JANUARY 2017.
On 13–15 October 2026, at the Palais des Congrès in Versailles, delegates to the General Conference on Weights and Measures vote on a more radical solution: retiring the leap second altogether, replaced by a far rarer "leap hour" that might not be needed for centuries.
"If we wait until 2035, we have a 30 percent risk of a negative leap second." Patrizia Tavella, Director of the Time Department, International Bureau of Weights and Measures
The 2035 deadline had already been set by a 2022 resolution. The risk threshold accepted by the institutions involved has, until now, been 10 percent. If the Versailles vote passes, the needle on the dial above will never again need to reach the red threshold on the right: the threshold itself moves, and the mechanism that reset the world's clock 27 times enters the archive, too.
Sources
Primary documents
- IERS Bulletin C, No. 72
- IERS EOP 20 C04, daily series 1962–present
- 28th General Conference on Weights and Measures (CGPM)
- Duncan C. Agnew, "A global timekeeping problem postponed by global warming," Nature, 27 March 2024
Secondary sources
Method note: the Earth-rotation series covers 1 January 1962 to 18 July 2026, the latest published by the Paris institute at the time of writing. The 2020s figures are calculated over the span elapsed so far, marked separately above, not a full decade.