The Leap Second Is Dead: Why 69 Countries Voted to Abolish It

Yesterday, at a conference hall outside Paris, representatives from 69 countries voted to abolish the leap second. The change takes effect on May 20, 2027, and it ends thirty years of debate over one of the strangest compromises in engineering: a single second, inserted into world time whenever the Earth wobbles off schedule, that has grounded airliners and panicked DNS servers for half a century.

This isn't a science story about clocks. It's a story about what happens when an irregular physical reality meets software that assumes time always moves forward. If you've ever written code that compares timestamps, you have opinions about this vote — whether you know it or not.

What a leap second actually is

We run on two clocks, and they don't agree. The first is UTC, Coordinated Universal Time, which ticks with the vibrations of cesium atoms. It's precise to a fault. The second is UT1, time as measured by the actual rotation of the Earth — the sun coming up, going down, doing it again.

The problem is that the Earth is a terrible clock. Its rotation speeds up and slows down under the influence of the oceans, the atmosphere, and torque sloshing around in the molten core. Long-term, the planet is slowing down — days get longer by roughly two milliseconds per century. But "long-term" hides the important part: short-term, the rotation is jittery, and in recent years it has been speeding up.

So whenever UTC and UT1 drift more than 0.9 seconds apart, the International Earth Rotation and Reference Systems Service announces a correction: at the end of June or December, clocks worldwide insert 23:59:60 — a 61-second minute. Since the system began in 1972, this has happened 27 times. The last was December 31, 2016. You never noticed, because the announcement happens months ahead and your phone just plays along. Your servers, historically, did not.

The one second that kept breaking the internet

Computers expect time to flow continuously. A second appearing out of nowhere — or worse, the clock reporting 23:59:60 — violates assumptions baked into kernels, databases, and every timestamp comparison ever written. The casualty list is long.

June 30, 2012. The leap second triggered a livelock bug in the Linux kernel that sent Java and Cassandra nodes into high-CPU spins across the industry. Reddit, LinkedIn, Mozilla, Yelp, and Foursquare all reported outages. Qantas's reservation system went down and more than 400 flights had to be rescheduled or grounded. The only mercy was that it happened on a weekend, so the stock markets were closed.

January 1, 2017. At midnight UTC, a leap second caused a subtle backward jump in the monotonic time reference inside Cloudflare's RRDNS software — their custom DNS resolver, written in Go. A variable tracking a time delta, which should always be zero or positive, went negative. The code panicked. Recovery logic caught the panic, but not before around 0.2% of DNS queries globally started failing — roughly 1% of HTTP requests across what were then 102 data centers. It took 90 minutes to patch the worst machines and over six hours to fully resolve. The root cause wasn't a bad deploy. It was one extra second, exposing the assumption that time always moves forward.

Big tech's answer was to stop telling the truth. Google invented the "leap smear": instead of letting servers meet 23:59:60 head-on, their time servers add a few milliseconds to every update across a window of hours beforehand. By midnight the adjustment is fully absorbed and no machine ever sees the odd timestamp. The idea came from SRE Christopher Pascoe, who laid it out on Google's blog in September 2011 — growing out of a 2005 scare, when some of Google's clustered systems stopped accepting work during that year's leap second. AWS and others followed. Meta smears the second across 17 hours. Stock exchanges smear it over about two hours before markets open; the NYSE has simply paused trading for an hour.

That workaround tells you everything: the world's biggest engineering organizations decided that lying to their computers about the time was safer than telling them the truth about the Earth.

The vote, and what actually changes

At the 28th meeting of the General Conference on Weights and Measures in Versailles on October 15, member states voted — not unanimously, but with the required majority — to pass a resolution introduced by Noël Dimarcq, president of the BIPM's Consultative Committee for Time and Frequency. The leap second dies on May 20, 2027, World Metrology Day.

It is replaced by the "leap hour." Instead of keeping UTC within a second of astronomical time, the new rules allow the two to drift apart by up to 3,600 seconds before any correction is applied. The BIPM's estimate: at the current drift rate, that correction won't be needed for over three thousand years. The problem is being handed, deliberately, to future generations.

The vote also accelerates a timetable everyone had already agreed to. Back in 2022, the same body voted to end leap seconds by 2035. Yesterday's decision pulls that forward by eight years. Something changed between 2022 and now — and it's the most interesting part of the story.

The real reason: the negative leap second

Here's the part that moved the vote. Earth's rotation has been accelerating in recent years. Timekeepers were looking at roughly a 30% chance that, within the coming decade, they'd need to do something no one has ever done: a negative leap second — deleting a second, skipping 23:59:59 straight to 00:00:00.

Every leap second in history has been an addition. Removing one is a different animal. Going backward in time is exactly the thing that crashed Cloudflare — and that was a tiny, accidental jump. A scheduled, global one-second reversal would hit every system that assumes time never retreats. Industry estimates are blunt: most digital timekeeping systems would probably crash.

That prospect is what united the tech industry. Google, Microsoft, Meta, and Amazon lobbied together — alongside the US National Institute of Standards and Technology and French timekeepers — to scrap the leap second entirely. Their argument, as one researcher put it, is that living with inaccuracy is safer than the inaccuracy of a correction. A French metrology official quoted by AFP said the vote was needed to avoid difficulties "which risked a major bug as early as next year."

Astronomers have a counter-argument worth hearing: loosening civil time from Earth's rotation means noon eventually isn't when the sun is overhead. The rebuttal is timescale. At the drift rates involved, the hour of slack won't be exhausted for millennia. Navigation systems like GPS already run on continuous internal time for exactly this reason. Sun-dial time has lost every practical battle it was ever in.

What this means for you

For almost every developer reading this: nothing breaks on May 20, 2027. UTC continues. Your NTP servers keep working. The leap smear machinery at Google, Amazon, and Meta quietly becomes unnecessary.

But there are two things worth carrying out of this story. The first is mechanical: if you write code that reasons about time, reason about monotonic clocks for measuring durations, and wall clocks only for showing humans what time it is. Every leap-second postmortem is the same bug in a different costume — somebody used the wall clock as a stopwatch.

The second is bigger. For 54 years, the world ran a system where the definition of a second was negotiated between atomic physics and a wobbling planet, corrected by announcement — and it worked, mostly, except when it catastrophically didn't. Yesterday, 69 countries voted to stop negotiating. The planet gets to wobble on its own. Our clocks get to tick on their own. And nobody's code ever has to handle 23:59:60 again.

References

• Nature — "World's timekeepers upend how they adjust time to avoid catastrophic 'negative second'" (Oct 15, 2026): https://www.nature.com/articles/d41586-026-03329-z
• Scientific American — "Scientists vote to end leap seconds, redefining how the world keeps time" (Oct 15, 2026): https://www.scientificamerican.com/article/scientists-vote-to-end-leap-seconds-redefining-how-the-world-keeps-time/
• CNN — "The leap second is on the chopping block — and timekeeping could change forever" (Oct 2026): https://www.cnn.com/2026/10/13/science/leap-second-elimination-time-standard
• Le Temps (AFP) — "Time stands still: the leap second is no more" (Oct 15, 2026): https://www.letemps.ch/en/sciences/physics-chemistry/time-stands-still-the-leap-second-is-no-more
• ChosunBiz — "CGPM replaces leap seconds with leap hour to stabilize global time" (Oct 16, 2026): https://biz.chosun.com/en/en-science/2026/10/16/WM7YRMF4INDOFNGCZEAVWNPIXM/
• Business Standard — "Every second counts. Or does it? The answer holds leap second's fate" (on the industry lobbying): https://www.business-standard.com/amp/article/international/every-second-counts-or-does-it-the-answer-holds-leap-second-s-fate-122111700748_1.html
• urandom.io — "Time, the invisible dependency" (Feb 2026; Cloudflare 2017 and 2012 postmortem details): https://github.com/urandomio/urandom.io/blob/HEAD/src/content/blog/2026-02-19-time-the-invisible-dependency.md