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. On 13–15 October 2026, in Versailles, CGPM delegates will vote on Draft Resolution C; if adopted, UTC would become continuous from 20 May 2027.
your clock, right now—
The mechanism
A dial reset twenty-seven times
Tides raise and lower the oceans, and their friction brakes Earth's spin, very slowly: the day lengthens by about two milliseconds each century. On top of that trend come winds, ocean currents and molten motion deep inside the planet, speeding it up or slowing it down on a schedule nobody can write in advance.
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. 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 personal instrument
How many of the 27 fell in your lifetime?
Pick your birth date; the page counts, entirely inside your browser, how many of the 27 leap seconds were inserted since then.
without a time, we count from midnight UTC
Choose a valid date before the present moment.
The time is read as UTC; the form doesn't ask for your birth timezone. The Moon comparison uses the speed of light (299,792.458 km/s) and the average Earth–Moon distance (384,400 km).
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 have been needed around 2026, while the meltwater pushed the projection to around 2029. A later CCTF/IERS assessment, developed after the 2025 workshop and published in the BIPM's July 2026 report, treats the risk probabilistically: about 30% for a negative leap second and 50% for a positive one in the next ten years. That makes 2029 a projection, not a fixed date.
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 will vote on Draft Resolution C. If adopted, continuous UTC would take effect on 20 May 2027, and the maximum |UT1−UTC| would rise from 0.9 seconds to 3,600 seconds, or one hour, enough to avoid adjustments for several centuries. "Leap hour" is journalistic shorthand for that tolerance; the draft does not schedule a future one-hour correction.
"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)
- CGPM 2026 Draft Resolutions
- CCTF report on Draft Resolution C
- 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.