To the world’s timekeepers, the leap second is a kludge, a bane, a pain in the little hand. Now they’re ditching it. Will our days ever be the same?

“The Astronomer,” a 1668 painting by the Dutch artist Johannes Vermeer. Credit... DeAgostini/Getty Images Smart Adaptor

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Roughly every four years, an extra day gets tacked onto the end of February, a time-keeping convention known as the leap year. The practice of adjusting the calendar with an extra day was established by Julius Caesar more than 2,000 years ago and modified in the 16th century by Pope Gregory XIII, bequeathing us the Julian and Gregorian calendars.

That extra day is a way of aligning the calendar year of 365 days with how long it actually takes Earth to make a trip around the sun, which is nearly one-quarter of a day longer. The added day ensures that the seasons stay put rather than shifting around the year as the mismatch lengthens.

Humanity struggles to impose order on the small end of the time scale, too. Lately the second is running into trouble. Traditionally the unit was defined in astronomical terms, as one-86,400th of the mean solar day (the time it takes Earth to rotate once on its axis). In 1967 the world’s metrologists instead began measuring time from the ground up, with atomic clocks. The official length of the basic unit, the second, was fixed at 9,192,631,770 vibrations of an atom of cesium 133. Eighty-six thousand four hundred such seconds compose one day.

But Earth’s rotation slows ever so slightly from year to year, and the astronomical second (like the astronomical day) has gradually grown longer than the atomic one. To compensate, starting in 1972, metrologists began occasionally inserting an extra second — a leap second — to the end of an atomic day. In effect, whenever atomic time is a full second ahead, it stops for a second to allow Earth to catch up. Ten leap seconds were added to the atomic time scale in 1972, and 27 more have been added since.

Adding that extra second is no small task. Moreover, Earth’s rotation is slightly erratic, so the leap second is both irregular and unpredictable. Fifty years ago, those qualities made inserting the leap second difficult. Today the endeavor is a technical nightmare, because precise timing has become integral to society’s highly computerized infrastructure.

“What was before just a way of measuring the flow of time is today essential for transportation, location, defense, finance, space competition,” said Felicitas Arias, former director of the time department of the International Bureau of Weights and Measures, known as B.I.P.M. from its French name and based outside Paris. “Time is ruling the world.”

The process of squaring these two time scales has become so unruly that the world’s time mavens have made a bold decision: to abandon the leap second by 2035. Civilization will wholly embrace atomic time; and the difference, or tolerance, between atomic time and Earth time will go unspecified until timekeepers come up with a better plan for reconciling the two. The vote, in the form of Resolution D, occurred on Friday at a meeting in Versailles of the Bureau’s member nations.

The resolution severs the timekeeping of atoms from the timekeeping of the heavens, probably for generations to come. The change will be indiscernible for most of us, in practical terms. (It will take a few thousand years for atomic time to diverge as much as an hour from Earth time.)

But the second is a huge amount of time in the technology of the internet. Cellphone transmissions, power grids and computer networks are synchronized to minuscule fractions of a second. High-frequency traders in financial markets execute orders in thousandths and even billionths of a second. By international law, data packages related to these financial transactions must be time-stamped to that fine level of precision, recorded and made traceable back to Coordinated Universal Time, the universally agreed-upon standard managed by the timekeepers at the B.I.P.M.

Every additional leap second introduces the risk of confusion: that some digital networks won’t implement the change correctly, won’t know precisely what time it is with regard to the other systems, and will fail to synchronize properly. The leap second is a dollop of potential chaos in a soufflé that demands precision.

For that reason, discarding the leap second had wide support from nations across the world, including the United States. The result of the vote was not a foregone conclusion, however. The fate of the leap second has long been the stuff of high diplomatic drama, designated one of just four “hot topics” at the B.I.P.M. Getting Resolution D on the agenda involved more than two decades of study, negotiation and compromise to resolve the issue.

“It should have happened 20 years ago, and if not for political maneuvering, it probably would have happened 20 years ago,” said Judah Levine, a physicist at the National Institute of Standards and Technology, or NIST, in Boulder, Colo. He is co-chair with Dr. Tavella of the B.I.P.M. committee that discusses hot topics, and he helped draft the resolution.

Russia, for instance, tried to delay a shift away from the leap second because doing so would require extensive alterations to its GLONASS satellite system, which incorporates the extra second. As a result, the resolution was phrased to postpone any change until 2035. The United Kingdom, historically and emotionally tethered to the astronomical standard, enshrined in Greenwich Mean Time, has long been reluctant to commit publicly, but was a firm supporter of the resolution in the end.

The fate of the leap second is more than just the fate of the leap second. It also affects Coordinated Universal Time, the international standard for timekeeping, which the leap second has been slowly undermining.

Coordinated Universal Time, or U.T.C., is tenderly constructed from readings made by atomic clocks kept at national laboratories around the world. These clocks tick off, or “realize,” their best seconds and send the measurements to the B.I.P.M. There, timekeepers painstakingly assemble the readings — averaging, weighting, adjusting for discrepancies — into an ideal second for everyone everywhere to agree on and employ, occasionally adding leap seconds as needed. This assembly process takes time. And so once a month the Bureau publishes the perfect time in the form of a newsletter, called Circular T, that tells each national clock how much it diverges from the international standard, to help it improve its aim the following month.

Coordinated Universal Time is the world’s official time scale, and will remain so even without the leap second. Global time zones are described in reference to it. (New York time currently is U.T.C. minus five hours.) And the beating heart, the second, is the most important in the constellation of standard measurements overseen by the B.I.P.M., alongside the meter (length), kilogram (mass), kelvin (temperature), candela (intensity of light), ampere (electric current) and mole (amount of substance).

The idea, formalized a century and a half ago by national signatories to an international treaty called the Meter Convention, is that each unit of measurement should be identical everywhere in the world; one meter in Spain is precisely one meter in Singapore. The seven standard units are integral to fair commerce, reproducible science and reliable technology. The second is extra-special because it underpins all the other units except the mole. For instance, the meter is defined as the distance light travels in a vacuum during one-299,792,458ths of a second, and the kilogram was recently redefined in terms of the second.

In addition, the second is tethered to a time scale, or flow of seconds. A key tenet of modern life is that not only must the unit of time be identical no matter where it is measured, so must the flow of seconds of which the one is a part.

But the leap second was putting that tenet at risk. The kludge is so technically difficult for digital technology to incorporate that other, ersatz methods of timekeeping — unofficial, but free of leap seconds and easier to implement — have begun to displace U.T.C., according to a recent article in the journal Metrologia. To supporters of Resolution D, removing the leap second from U.T.C. will make the standard time scale friendlier to modern digital technology, at least in the century following 2035. Coordinated Universal Time will still be universal, just not coordinated with Earth time.

“There is this problem we want to stop, which is this proliferation of pseudo time scales, because they are not time scales in the metrological sense,” Dr. Arias said.

The time scale most commonly used in place of U.T.C. is the American government’s global-positioning satellite system, or GPS. Each satellite in the GPS network, which is operated and maintained by the U.S. Space Force, carries atomic clocks that provide time data, along with information about longitude, latitude and altitude.

Users of GPS, which include cellphone and data networks, can determine the time of day to within 100 billionth of a second, and the information is free and widely available. But it is neither funneled through the B.I.P.M. nor adjusted for leap seconds. The United Nations’ International Telecommunication Union recently suggested that telecommunication networks make GPS, rather than U.T.C., their official time because it does not incorporate leap seconds and therefore is an uninterrupted flow of time.

To metrologists the implications are grave: Although GPS keeps good time, using it rather than U.T.C. would mean that time would no longer be overseen by an organization that must abide by international agreements.

“The increasing use of signals from the GPS satellites effectively means that the U.S. military controls a primary source of international time signals with almost no oversight nationally or internationally,” noted the Metrologia article, which was written by Dr. Levine, Dr. Tavella and Martin Milton, the director of the B.I.P.M.

Moreover, the clocks aboard satellites are inconsistent across systems. Russia’s GLONASS runs on U.T.C. (adjusted by three hours) and leap seconds, but the other satellite navigational systems do not, and they diverge from universal time by different amounts, depending on when they became operational. GPS and Galileo, the European system, are 18 seconds ahead of U.T.C. The Chinese system BeiDou is four seconds ahead. They each function well because they are internally consistent and because their divergence from U.T.C. can be tracked, but they are not traceable back to U.T.C.

Even computing systems that currently insert the leap second do so in different ways. As a result, the time stamps required for commercial and financial transactions are sometimes out of whack during the adjustment period, risking system crashes and an occasional lack of traceability. Google smears the extra second across a whole day, while Meta, Alibaba and Microsoft each add the extra second in their own bespoke way. And according to the Metrologia paper, the number of errors in implementing the leap second has been increasing over time.

“It is anarchy,” Dr. Tavella said.

An additional wrinkle looms. The leap second has been necessary because atomic time runs faster than Earth time. But that is changing: Earth’s rotation rate began speeding up right around the time the leap second was invented. This month or next, Earth time will catch up to atomic time. By about 2030, if the trend persists, Earth time will overtake atomic time by about a second — so metrologists will have to insert a negative leap second to keep the two time scales in sync.

In effect, a second will vanish. Such an experiment has never been tested on computer systems, and many metrologists fear a digital disaster. “The first time in the history of U.T.C. that a negative leap second occurs, and nobody knows what to do,” Dr. Arias said.

Time is fraught with emotion. Consider the messy debates that have erupted recently over whether to keep daylight saving time.

Last month, Mexico’s Senate voted to end the practice but only for parts of the country not sharing the border with the United States. In March, the U.S. Senate voted to permanently keep daylight saving time, but the motion is stalled in the House. In the European Union in 2018, Parliament voted to keep clocks the same year-round but has been flummoxed in how to do so or which setting to choose.

The leap second, though less visible to the public, also elicits strong opinions. The Vatican, for instance, has argued for keeping the leap second, on existential grounds. Time “is a constant reminder of our mortality,” wrote the Rev. Pavel Gabor, an astrophysicist and the vice director of the Vatican Observatory Research Group in Tucson, Ariz., in “The Science of Time,” published in 2017.

“And perhaps because of this we want to believe that our time, our lifetime, somehow corresponds to the eternal cosmic cycles.”

Dr. Tavella and her colleagues consulted with Dr. Gabor recently as they sought to navigate the implications of suspending the leap second. He counseled them to remember that the “ancient and sacred” task of timekeeping has always been laden with compromise.

The Julian calendar, with its original leap year additions, was 11 minutes and 14 seconds longer than the actual year, adding up to an extra 10 days by the time Pope Gregory XIII restructured the calendar and removed the extra days. His fix was to subtract three leap days every 400 years — a formula that needs correction only every few thousand years. New information invites new solutions.

Dr. Tavella was adamant that whatever the outcome of Resolution D, time would retain its ancient link to the stars. “We are not abandoning the rotation of the Earth,” she said. “We know the relationship between atomic time and the rotation of the Earth.” The differences would continue to be calculated and made available, just not actively implemented.

Digital Electronic Timer Of course, even with Resolution D passing, future generations of timekeepers will continue to try to reconcile atomic time with celestial time — perhaps with a leap minute, which will be called for in about a century, or eventually a leap hour, or something not yet imaginable.