The General Conference on Weights and Measures opens on Tuesday, October 13. The setting is the Palais des Congrès de Versailles, a short walk from the palace that Louis XIV built under the emblem of the sun. Delegates from the states party to the Metre Convention will spend three days on the business of measurement. Near the close of the meeting, at the twenty-fifth item of the agenda, they will vote on a paper called Draft Resolution C. The paper proposes that on May 20, 2027, the world’s civil clock stop answering to the sky. Resolutions reach this conference after years of committee negotiation, and they are usually adopted as drafted. If this one carries, a deference that began on June 30, 1972, with the first leap second, comes to an end.
Draft Resolution C does one technical thing. It widens the permitted gap between Coordinated Universal Time, the atomic time scale that runs the world’s networks, and UT1, the time kept by the turning Earth, from 0.9 seconds to 3,600 seconds, one full hour. UTC has been pinned within 0.9 seconds of the planet’s rotation since 1972, and each time the gap approached the limit, the timekeepers inserted a leap second to close it. The new rule lets the gap widen untouched for centuries, and it retires the instrument that has tied the clock to the sun for fifty-four years. The rule change only moves the fence; once the fence stands an hour out, the corrections that kept crashing the networks fall harmlessly inside it. The committee chose the date with a sense of ceremony: May 20 is World Metrology Day, the anniversary of the signing of the Metre Convention in 1875. No hour will be added or dropped that morning.
The reason for the hurry is a report. It came from the Consultative Committee for Time and Frequency, sent to the conference this year. The Earth’s rotation slowed for decades, and all 27 leap seconds since 1972 have been additions. Since 2020 the planet has been spinning faster, and the shortest days in the atomic record have fallen in this decade, among them a day in June 2022. If the acceleration holds, the timekeepers face a correction no clock has ever executed: a negative leap second, a minute of fifty-nine seconds, in which the clock jumps from 23:59:58 to midnight, and the second named 23:59:59 never exists. Three independent research teams built stochastic models of the length of the day, validated them against centuries of rotation measurements, and reached what the report calls very similar results. Their estimate puts a 30 percent probability on a negative leap second within ten years, and a 50 percent probability on a positive one.
Patrizia Tavella, the director of the Time Department at the International Bureau of Weights and Measures, warns that a negative leap second has never been applied or tested, and that the problems it could create are without precedent. The networks have been patched, sometimes at high cost, for the addition. They have never been asked to count backward on a live network. Distributed databases order transactions by timestamp. Audit logs assume time moves one way. A scheduler that asks how long remains until its deadline can receive a negative number. The committee asked its users and sister organizations what odds they could accept, and the answer came back that even a 10 percent risk was too much. The standardization arm of the International Telecommunication Union warned that legacy systems would not handle the event properly. Industry statements recorded in the report compare the required preparation to the Y2K conversion. The engineers responsible for the Precision Time Protocol, which synchronizes telecommunications, finance, and power grids to millionths of a second, told the committee that their applications require a continuous time scale. The vote at Versailles is the escape. With the fence an hour out, whenever the planet demands the subtraction, nobody has to do anything at all.
Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography at the University of California, San Diego, published a study in Nature on March 27, 2024, under a title that carries its finding on its face: “A global timekeeping problem postponed by global warming.” Meltwater pouring off Greenland and Antarctica shifts mass toward the equator, and a planet wider at the waist spins slower, as a skater slows with arms extended. Agnew’s calculation showed that without the accelerated melting of the ice sheets, the Earth’s quickening spin would have forced the first negative leap second as early as 2026. The meltwater braked the planet enough to push the reckoning to 2029 or later. The ice that is drowning the coasts is shielding the networks, and nobody arranged that. Predictions of the Earth’s rotation hold for a few years and no farther; a few years ago they ran in the opposite direction.
Until 1960 the second was the fraction 1/86,400 of the mean solar day, and the astronomers owned it because the sky owned it. The eleventh General Conference redefined it in 1960 as the fraction 1/31,556,925.9747 of the tropical year 1900, a unit drawn from the Earth’s orbit around the sun, and the sky owned it still. Louis Essen built the first practical cesium clock at Britain’s National Physical Laboratory in 1955. Essen and William Markowitz of the United States Naval Observatory then spent nearly three years comparing the cesium frequency against the astronomical second, timing the comparison off shortwave time signals across the Atlantic. The number their work produced, 9,192,631,770 cycles of the cesium-133 atom’s chosen transition, became the definition on October 13, 1967, when the thirteenth General Conference voted the sky out of the standard. The delegates who open the twenty-eighth conference this October 13 convene fifty-nine years to the day after that vote.
An arrangement in 1972 set the covenant. International Atomic Time, the pure count of cesium seconds, ticks undisturbed, a weighted average of more than 450 atomic clocks in some 80 laboratories, computed each month at Sèvres outside Paris and published in a bulletin called Circular T. Coordinated Universal Time was built to follow it, cesium second for cesium second, with one concession: whenever UTC drifted 0.9 seconds from the turning Earth, a leap second would pull it back. The first came at the end of June 30, 1972, a minute containing a second named 23:59:60, and 26 more followed at intervals from six months to seven years, the last on December 31, 2016. The International Earth Rotation and Reference Systems Service, which watches the planet against the fixed quasars, announces each insertion about six months ahead in a document called Bulletin C. Atomic time now runs 37 seconds ahead of the civil scale, ten seconds of inherited offset plus 27 leaps.
A leap second at the end of June 2012 triggered a Linux kernel flaw across the Internet. The Amadeus Altea reservation system went down for an hour. Qantas and Virgin Australia checked in passengers by hand, and travelers learned that morning how thin the floor under the schedule is. Reddit, LinkedIn, Mozilla, and Yelp fought their own servers back to life. Google had already refused to leap, smearing each extra second across the hours around it, and its rivals had to accommodate the smear. The Global Positioning System refused from the start. GPS time began on January 6, 1980, synchronized to UTC, and it has never leaped; it runs 18 seconds ahead of the civil clock, a private time orbiting overhead while the ground adjusts. European regulators now require high-frequency trading systems to trace their timestamps to UTC within 100 microseconds, a rule that turns every insertion into an audit exposure. Each leap is announced half a year ahead, and each arrives as a small emergency anyway.
Civil time was solar time for all of human history, and the sun still schedules the tides and the crops. Celestial navigators and observatories use UT1 to this day. Five countries, among them the United States, the United Kingdom, Canada, Russia, and Argentina, still broadcast the difference between clock and sky under radio regulations that permit the code until 2035. The claim fails on arithmetic. Under the widened fence, the civil clock drifts from the sun by roughly a second every few years; a century from now noon may lag the sun by a minute or two, and no lunch will move. The astronomers keep their corrections, published and precise. What they lose is the power to stop the world’s clock.
I spent the winter of 1990 working as an assistant to the playwright Arthur Kopit in New York. My job was pages and doors. I carried rewrites from his desk to the rehearsal room, and one night that winter I seated Arthur Miller in the last row of the theater, where the famous man could watch the play like anyone else. In the theater nobody asks where the sun is. Curtain is eight o’clock in January and in June. The house lights dim on a number agreed upon months before, and the number holds because everyone in the building keeps it. UTC after May 20, 2027, becomes the same kind of instrument, a covenant among the people and machines who keep it, answerable to its own arithmetic. The theater taught me that such a covenant is enough. Curtain time has never needed the sky’s permission, and a civilization that runs on timestamps can decide the same about its seconds.
The conference will also take up a companion paper, Draft Resolution D. It asks the delegates to define an international lunar reference time scale, a unique scale traceable to UTC. The need is relativistic: a clock on the Moon runs fast by about 56 microseconds a day under lighter gravity, and 56 microseconds is time enough for light to cross more than ten miles. The International Astronomical Union voted its lunar framework in 2024, and Washington has ordered a Coordinated Lunar Time strategy delivered by the last day of 2026. A third paper, Draft Resolution B, prepares the redefinition of the second itself, a handover from cesium to optical clocks so steady they would not slip a second across the age of the universe. The program hangs together end to end. It unties the civil clock from this sky, and it bolts a clock to the next one. The sharpened second will serve both.
All three claimants can now read the verdict in the drafts. The astronomers lost the unit in 1967 and kept a leash on the clock through the leap second, a leash held for fifty-four years. The engineers paid the toll in crashed servers and audit rules, and they have declined to keep paying. As for the sun: it keeps the day and the tides, and it forfeits the timestamp. The committee’s own phrase is the long-term continuity of UTC. No 23:59:60, no minute of fifty-nine seconds; corrections cease until the drift approaches an hour. That moment sits centuries away, and another conference can then ask whether anyone alive still sets a watch by the sky. If the delegates amend the paper or lay it aside, the 2022 decision stands beneath it, requiring the fence to move by 2035.
Louis XIV chose the sun as his emblem. Everything in his kingdom was to revolve around his person, and Versailles was the proof. Three and a half centuries later, in a congress hall a short walk from his gates, the delegates of the Metre Convention will decide whether the world’s clock keeps revolving around the actual sun. The result will follow by press release after the final gavel, and the expectation in the time community is adoption as drafted, effective May 20, 2027. The palace is a museum now, and the emblem above its gates has been demoted to scenery. On May 21 of next year, if the vote has carried, the sun will rise over Versailles at its accustomed hour, and the phones in every pocket will show a time that owes the sun nothing. That is the whole question on the table: whether the difference between the sun and the clock is anyone’s business anymore.
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