Long read · about 12 minutes

The Last Leap Second

For half a century, the world's timekeepers bargained with a planet that won't keep perfect time. This fall they walked away from the table, and showed how human our idea of time has always been.

By the onlineclock.app team · Published October 20, 2026

At the end of December 31, 2016, Coordinated Universal Time did something clocks are never supposed to do. Instead of rolling over from 23:59:59 to midnight, it counted one extra second, a moment that never appears on an ordinary watch face: 23:59:60.

For one extra second, the world's atomic time standard made room for the Earth's uneven spin.

Nobody knew it then, but that was the last one. It was the 27th leap second since 1972, and none has been added since. On Thursday, October 15, 2026, at Versailles, the 28th General Conference on Weights and Measures (CGPM) voted to end the practice. Under its Resolution 5, continuous UTC takes effect on May 20, 2027. The allowed gap between UTC and UT1, the time scale that follows the Earth's rotation, widens from under a second to 3,600 seconds: a full hour, enough to keep UTC running without a break for several centuries. The vote was not unanimous. Russia voted against and Belarus abstained.

When the new rule takes effect, clocks won't jump. No alarm will sound, and most people will notice nothing at all. Still, something old will have ended: a fifty-year arrangement in which humanity's most precise machines agreed, every so often, to wait for the sky.

To see why that matters, it helps to know that two kinds of time are involved here, and they have never fully agreed.

The planet's clock

The first kind of time is the one we inherited: the turning of the Earth, the sun climbing to its highest point, the long swing of the seasons. For most of human history, the sky was the final reference for every clock. Noon was simply the moment the sun stood highest over your town. Walk a few miles east or west, and noon came at a different moment.

The trouble is that the planet keeps poor time. Tides raised by the Moon act like a brake, so over centuries and millennia the average day slowly gets longer. Over shorter spans the Earth speeds up and slows down as the atmosphere, the oceans and the molten core trade momentum with the solid planet. On July 5, 2024, the Earth finished a rotation about 1.66 milliseconds short of 86,400 seconds, the shortest day since atomic clocks began keeping score.

A day, in other words, is not a fixed thing. It's a measurement of a spinning rock that can't quite hold a steady pace.

The atom's clock

The second kind of time is the one we built.

In 1967 the 13th CGPM cut the second loose from the sky. From then on, the official second was no longer a fraction of a day. It became 9,192,631,770 cycles of the radiation from a particular transition in the cesium-133 atom (how a second is defined). That oddly specific number was chosen so the new second would match the old one, the "ephemeris second," which had been defined from the Earth's orbit around the Sun. The change was meant to be invisible.

Philosophically, though, it was a revolution. A cesium atom doesn't slow down when the Moon tugs at the oceans, and it doesn't speed up in summer. Under carefully controlled conditions, cesium atoms in any laboratory give the same frequency. For the first time, humanity had a clock steadier than the planet it lived on.

That's when the trouble started.

A patch on a patch

If one clock runs steadily and the other wanders, the two drift apart. When the modern system began in 1972, UTC was set 10 seconds behind International Atomic Time (TAI). The deal was that UTC would tick at the atom's rate, but whenever it drifted close to 0.9 seconds from the Earth's rotation, an extra second would be slipped in at the end of June 30 or December 31. The clock would read 23:59:59, then 23:59:60, then midnight. Every leap second so far has been an added one.

For wall clocks and wristwatches it hardly mattered. In a world run by software, it became a hazard.

A great deal of software assumes that every minute has exactly 60 seconds and that the clock never repeats itself. A 61-second minute breaks that assumption. On the night of June 30, 2012, it broke a lot of things. Reddit, Mozilla, LinkedIn and Yelp all had problems, many traced to a bug in the Linux kernel. The Amadeus reservation system that serves many airlines went down. In Australia, Qantas and Virgin Australia staff checked passengers in by hand, and more than 400 Qantas flights were delayed by at least two hours.

Then the planet changed the stakes. Because the Earth has recently been spinning a little fast, timekeepers began to discuss something that had never happened: a negative leap second, when the world's clocks would skip straight from 23:59:58 to midnight. A March 2025 workshop of the international time and Earth-rotation experts (the CCTF and the IERS) put the chance of one at about 30% by 2035. The 2026 resolution itself warns that a negative leap second has never been applied and poses a high risk to critical infrastructure that is largely unprepared for it.

Faced with a planet that won't keep steady time and systems that can't safely absorb a missing second, the timekeepers chose to give UTC more room to drift.

Time has always been a decision

It's tempting to see the end of the leap second as a footnote for engineers. History says otherwise. Each time people have changed their clocks or calendars, they have also been deciding whose needs time should serve.

In 1582, a pope removed ten days. The old Julian calendar had slowly drifted against the seasons, pulling the date of Easter away from the spring equinox. Under Pope Gregory XIII's reform, in the Catholic lands that adopted it first, Thursday, October 4 was followed by Friday, October 15. The days of the week carried on without a break; the dates in between simply never appeared. Britain and its colonies held out until 1752, by which time they had to skip eleven days (the Gregorian calendar, explained).

In 1883, American railroads took noon away from the towns. Before then, towns set their clocks by the sun and the railroads juggled dozens of local times. At noon on November 18, 1883, the railroads switched to standard time zones, and places east of their new zone's center line heard noon twice: "the Day of Two Noons." Chicago's clocks went back about nine and a half minutes. Congress didn't write the zones into law until the Standard Time Act of 1918 (who invented time zones?).

In 2011, a nation skipped a Friday. At the end of December 29, Samoa and neighboring Tokelau jumped straight to December 31, crossing to the other side of the International Date Line to line up their working week with Australia and New Zealand. It was Samoa's second switch: in 1892, to help trade with the United States, it had moved the other way and lived July 4 twice. Samoa's government decreed that workers would still be paid for the missing day, and that banks could not charge interest for it.

A pope, the railroads, a trading nation, and now a conference of measurement scientists. Each looked at the gap between the sky and the schedule and decided which one would give.

The clock inside the clock

Here's the strangest part. Even with the leap second gone, physics offers no single master clock that every observer would agree on, whatever their speed or altitude. Einstein showed that, and our machines confirm it every day (does time really slow down?).

GPS satellites orbit about 12,500 miles up, each carrying atomic clocks. Because they move fast, special relativity says their clocks should fall behind clocks on the ground by about 7 microseconds a day. Because they sit higher in the Earth's gravity, general relativity says they should run ahead by about 45 microseconds a day. The net result is a gain of roughly 38 microseconds every day. Left uncorrected, that timing error would make GPS positions wrong by roughly ten kilometers more each day. The map in your car works because engineers built it around the fact that time passes at different rates in different places.

The effect doesn't need an orbit. In 2010, physicists at the US National Institute of Standards and Technology raised one of a pair of ultra-precise aluminum-ion clocks by just 33 centimeters, about a foot, and watched it run faster. Over a 79-year lifetime the difference adds up to about 90 billionths of a second. That's far too small to feel, but it's real: you age a little faster when you climb a couple of steps.

And then there is the time we live inside, which no atom governs. In the Andes, the Aymara language uses nayra, a word tied to "eye" and "front," for the past, and qhipa, "back" or "behind," for the future. In a 2006 study of language and gesture, the cognitive scientist Rafael Núñez and the linguist Eve Sweetser found that Aymara speakers, especially older people who spoke little Spanish, gestured forward when talking about the past and back over the shoulder when talking about the future. The past, which you have seen, lies ahead in plain view. The future, which no one can see, comes up from behind.

Next to the steady beat of a cesium atom, that can look like a contradiction. It's really a reminder that the way we picture time is not the same as the way we measure it.

The question we left for later

When continuous UTC begins on May 20, 2027, UTC and the Earth's rotation will be free to drift apart, with no small corrections to pull them back. The gap will grow by fractions of a second, then by seconds, then perhaps by minutes, at a pace no one can predict precisely because it depends on the Earth itself. The new rules allow it to reach an hour, a margin meant to last for centuries.

The decision is practical and probably wise. But it doesn't settle the old argument between the planet's clock and the atom's clock. It postpones it. Far in the future, if the gap grows large enough, people will have to decide again whether and how to bring their clocks back in line with the sun. It's the question every generation before them faced: whose time is it?

That they'll be able to ask it at all says something about what time is. Time is something we measure; the time on our clocks is also something we agree on, and that agreement has always been provisional. It was rewritten by popes and railroads, renegotiated by island nations, and now relaxed by a vote in Versailles.

Look at any clock today, including the one on our homepage, which checks itself against internet time. Behind it lies a global system of atomic clocks, national laboratories, and satellite and fiber links, all feeding the UTC that the International Bureau of Weights and Measures computes. That everyday agreement is the end point of this whole long story, and a reminder of how much work it takes for billions of people to share a single "now."

Behind every one of those clocks is the thing itself, which no clock has ever fully captured. Time lets the same world be different without contradicting itself: the acorn and the oak, the child and the elder, the star and its ash. It runs a little faster on a mountaintop than at sea level. We remember the past and not the future, and the world changes in ways that never run backward.

We count it with shadows, pendulums and atoms. But clocks don't hold time; they only count it. Time is the one thing everyone is given and no one gets to keep, and that's why anything is precious.

Sources

Checked October 2026. Spotted a mistake? Tell us.

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