Roughly every few years starting in 1972, a one-second adjustment has been applied to the timekeeping systems running computers, financial networks, and communications infrastructure. A vote among the body responsible for coordinating global time standards resulted in a decision to eliminate the periodic adjustment long at the centre of debate. As of 2027, the new provisions are scheduled to come into force.
Delegates gathered under the banner of the International Bureau of Weights and Measures — known by its French acronym BIPM — voted at a conference held in France to retire the leap second, a periodic adjustment that had long been used to keep timekeeping systems in step with the planet's rotation. Much like the way calendars absorb an extra day every four years to stay aligned with Earth's path around the sun, this approach followed the same underlying logic. The calendar places a leap year on a fixed four-year cycle, a pattern astronomers and timekeepers have long documented.
Leap seconds exist to account for irregularities in Earth's rotational speed, and timekeepers have long grappled with managing them — a challenge that has taken on added weight now that the planet's spin appears to be accelerating, raising the prospect of a first-ever subtraction rather than addition to the clock. Noel Dimarcq, who chairs the BIPM's committee for time and frequency, noted in a statement that the timing of a recent international decision on the matter had become particularly apt given that shifts in Earth's rotation had introduced the possibility of a negative leap second — an adjustment with no historical precedent that could pose risks for infrastructure systems dependent on precise timekeeping.
Rather than inserting an extra second, timekeepers will hold off until a full hour's adjustment becomes necessary to realign atomic clocks with the rotational pace of Earth. Patrizia Tavella, who heads the bureau's time department, described the updated approach as more straightforward to monitor.
No firm timeline exists for when an extra hour might be inserted into the timekeeping system, though experts at a recent meeting noted the adjustment likely lies far beyond any foreseeable horizon. Andrew Novick, an electrical engineer who oversees the time and frequency division at the National Institute of Standards and Technology in Maryland, told attendees the interval before such a leap hour arrives could stretch into the thousands of years. Duncan Agnew, a geophysicist at the Scripps Institution of Oceanography in California, pointed out that for most of recorded history the planet itself served as humanity's most reliable measure of time.
Tracking time has long centred on subdividing the planet's rotational cycle — a spin that averages around 1,000 mph (1,600 km/h), yielding the familiar 86,400-second unit known as a day. Demand for finer timekeeping grew across the latter half of the 20th century, driven by the rise of computing technology, globally linked communication networks, and the push to send spacecraft beyond Earth's atmosphere.
Developed in 1963 as a worldwide standard for timekeeping, Coordinated Universal Time — known as UTC — grew out of atomic clock technology, which registers the passage of time by detecting the periodic shifts atoms undergo at fixed intervals. As Novick put it, a clock's true precision only becomes apparent once a more accurate one comes along to serve as comparison.
Gravitational pulls from neighbouring celestial bodies, along with the shifting of air masses, water, and rock across a planet's surface, can alter how fast that planet spins. Advances in timekeeping precision brought Earth's minor rotational irregularities into measurable relief, a development that led to the introduction of the leap second as a corrective tool.
Two technical difficulties arose for digital infrastructure as a result of the added second over the span of those years. Adjusting a clock by even one second introduces procedural complexity and carries the possibility of brief outages in systems that rely on precise timekeeping. Certain technology firms — Google among them — handled the adjustment differently, gradually nudging their system clocks to distribute the extra second across a period of time rather than inserting it all at once.
At times, the planet's spin actually runs ahead of schedule. Those same physical dynamics are capable of running in the opposite direction, which raises the possibility that a negative leap second could eventually be introduced. Systems built around UTC generally handle a forward clock adjustment without difficulty, yet the engineering required to process any reversal of that reference point reaches far fewer devices.
Gradual acceleration in the planet's rotational rate has been under way since roughly the early 2000s, tied to shifts occurring both deep within Earth's core and at its surface — including the ongoing loss of mass from polar ice. That trend raises the probability of a negative leap second becoming necessary, and it played a part in the decision by those responsible for global timekeeping to phase out the practice altogether.
Dissenting voices were present among those who cast ballots. Leap-second accommodation built into their satellite infrastructure led the Russian delegation to oppose the measure, while representatives from Belarus withheld a definitive vote either way.
At present, the gap between atomic timekeeping and Earth's rotational pace sits near zero, which means a negative leap second was a genuine possibility, according to Novick. Novick and fellow researchers noted they would lose something they valued about the old approach to measuring time — the quirk that reliably drew public attention to their work, with Novick observing that it tended to bring NIST into news coverage precisely because it prompted people to reflect on how the Earth rotates and on the nature of time itself.
