Twenty-seven. That is how many leap seconds the world’s timekeepers have inserted into civil time since the system began in 1972, and it is the number at stake on Thursday, when delegations meet near Paris to decide whether to stop adding them for good. As Al Jazeera reported, scrapping the leap second would disconnect the clocks we use from the Earth’s rotation and realign the world’s timescales.
Why a second needs inserting at all
Civil time runs on two clocks that do not agree. The first is atomic: Coordinated Universal Time (UTC), kept by several hundred atomic clocks, in which a second is defined by the oscillation of a caesium atom and never varies. The second is astronomical: UT1, the time told by the Earth’s actual spin. The world’s clocks are designed on the assumption that the Earth completes one rotation in exactly 24 hours; it does not. The planet wobbles, and its rotation is gradually slowing, so the two scales drift apart by roughly a second every year or two, on no predictable schedule.
Since 1972 the fix has been the leap second: when UT1 and UTC drift 0.9 seconds apart, an extra second is inserted, so that a minute somewhere in the world reads 23:59:60 before midnight. That keeps atomic time within 0.9 seconds of solar time, which is what astronomers, navigators and anyone pointing a telescope cares about. It is also an irregular, unscheduled intervention in the reference clock that the internet, power grids and financial markets all synchronise to.
The decision already taken
Thursday’s debate is not starting from zero. In November 2022, the General Conference on Weights and Measures — the 59 member states of the International Bureau of Weights and Measures — adopted Resolution 4, which decides that the maximum permitted difference between UT1 and UTC will be increased “in, or before, 2035” and asks that a new maximum be chosen to guarantee the continuity of UTC “for at least a century”. In plain terms: the member states voted to let atomic time run free of the Earth’s rotation from 2035 at the latest, for decades and possibly for a hundred years or more. Thursday’s meeting near Paris is where the details get fought over.
The engineering case against the leap second is outage history. Every insertion is a one-off event that buggy code handles badly, and over the years leap seconds have knocked over systems at airlines, stock exchanges and large web companies. The counterargument is that decoupling civil time from the Sun is a one-way door: without adjustments, the difference between clock noon and solar noon grows, slowly but without limit. Some timekeepers have floated a “leap minute” or “leap hour” — a single correction every few centuries — as the compromise, letting UTC run continuously between rare, schedulable adjustments.
The rotation that will not hold still
The arithmetic has recently reversed sign. For decades the Earth’s spin slowed, which is why all 27 leap seconds so far have been additions. But in recent years the rotation has sped up, and researchers have warned that the first negative leap second — deleting a second, something no deployed system has ever had to do — could be needed as early as 2029. That is a projection, not a measurement, and it comes with a wrinkle: melting polar ice redistributes mass toward the equator and slows the spin, delaying the crossing point. Climate change, in effect, is buying the timekeepers time they are voting to stop using.
A negative leap second is the strongest technical argument for scrapping the mechanism entirely. Software has failed at adding a second after 27 rehearsals; no one has rehearsed subtracting one. If the delegates accept that argument, the 0.9-second cap becomes a much larger tolerance, UTC drifts free of UT1, and the question that settles the debate shifts from astronomical to administrative: how many seconds of drift the next century is willing to accumulate before anyone has to fix it again.