The Minute with Sixty-One Seconds

At the end of June 30, 1972, the world’s clocks were instructed to count like this:

23:59:59
23:59:60
00:00:00

The middle line was not a misprint. For one minute, the last day of June had 61 seconds. A minute later the calendar had advanced normally, as if time had not just acquired an extra room and then closed the door behind it.

That odd second was invented to settle an argument between two clocks. One clock was made of atoms and cared nothing for sunrise. The other was the Earth itself, rotating under the Sun, never at quite the same speed twice. Civil time was asked to satisfy both. It did so by occasionally stopping to let the planet catch up.

The clock that refuses to wobble

For most of human history, the day was the natural clock. Noon was when the Sun crossed a local meridian; midnight was the halfway point between one noon and the next. The arrangement was imprecise, but the sky supplied the reference. A sundial could be wrong by minutes and still be telling the same kind of time as an observatory.

Atomic clocks changed the temperament of the second. The modern SI second is defined by 9,192,631,770 cycles of radiation associated with cesium-133. That number was chosen from astronomical measurements in the late 1950s, but the object that now realizes the unit is an atom, not the turning Earth. Atomic transitions provide a remarkably stable interval. The planet does not.

Earth’s rotation speeds up and slows down for reasons that include movements in its atmosphere and oceans, exchanges of angular momentum inside the planet, and the long-term braking effect of the Moon’s tides. The changes are tiny. A day can gain or lose milliseconds. That is enough. Accumulated over months, the difference between a uniform atomic clock and the irregular rotation of the planet becomes a visible disagreement about where the sky ought to be.

The solution is Coordinated Universal Time, or UTC. It runs at the steady rate of atomic time, but is occasionally adjusted so that it remains within 0.9 seconds of UT1, the time scale derived from Earth’s rotation. The adjustment is a leap second. It does not make the atomic second longer. It adds one second to a particular minute so the civil clock remains close to the Sun’s position.

A compromise made of numbers

The first leap second was inserted at the end of June 1972. It was not the only adjustment that year: UTC began with a ten-second difference from International Atomic Time, reflecting corrections built into the new system. After that, timekeepers could make smaller repairs, one second at a time, when observations showed that the Earth was drifting too far from the atomic schedule.

The decision belongs to the International Earth Rotation and Reference Systems Service, or IERS. It watches the planet with astronomical measurements and announces its decision in a document called Bulletin C. Leap seconds are preferred at the end of June or December, giving engineers a predictable place to put an unpredictable event. They cannot be scheduled decades ahead because Earth’s rotation cannot be forecast that far with the needed precision.

The official notation is wonderfully literal. A positive leap second makes the final minute of a day run from 23:59:59 to 23:59:60 and then to 00:00:00. It is not a second stolen from the following day. It is a second inserted before the date changes.

There could, in theory, be a negative leap second. If the Earth rotated fast enough, the sequence would skip 23:59:59 and move from 23:59:58 to 00:00:00. That has never happened. Every leap second since 1972 has been positive, although recent changes in the planet’s rotation have made the hypothetical subtraction less remote than it once seemed.

By the end of 2016, there had been 27 leap seconds. The last one was announced in a short, bureaucratic notice from Paris. It instructed time authorities to use the sequence 23:59:59, 23:59:60, 00:00:00 at the turn of the year. Since then, the Earth has stayed close enough to atomic time that no further second has been required.

The second computers cannot quite see

A person can live through a leap second without noticing. A digital system has a harder time. Many clocks represent time as a count of seconds from an origin, then convert that count into hours and minutes. The ordinary format has no comfortable place for a second labeled 60. A machine may repeat 23:59:59, smear the extra second gradually across a day, or follow a special protocol. Different choices can make two systems disagree about which event happened first.

That ambiguity matters wherever timestamps are used to order transactions, coordinate machines, or compare measurements. The leap second is tiny, but it is not merely decorative: it creates a discontinuity in a time scale that software usually expects to advance in equal steps. During a leap second, a time server may receive the same conventional timestamp twice, with no simple way for a client using that format to tell the two instants apart.

This is why a compromise designed to keep clocks friendly to astronomers can be unfriendly to networks. Satellite navigation, telecommunications, financial systems and electrical grids all depend on clocks that agree. A clock that occasionally contains an extra label is not impossible to manage, but it is an awkward guest in systems built on the assumption that every minute has sixty seconds.

The argument is therefore not between people who care about the sky and people who care about computers. It is between two legitimate meanings of “correct.” Astronomers need a civil time that stays near the Earth’s angle in space. Engineers need a scale whose seconds are uniform and whose labels never repeat. UTC has been asked to be both, and the leap second is the seam where the requests meet.

When midnight lets go of the stars

In 2022, the General Conference on Weights and Measures agreed that the permitted difference between UTC and Earth’s rotation should be increased by or before 2035. The reason was practical: leap seconds create risks for digital infrastructure, and a negative leap second would be especially difficult because no one has ever had to insert one. The decision does not make the Earth more regular. It changes how much disagreement civil time will tolerate.

That is a quiet historical shift. The calendar will not suddenly lurch away from the Sun. The difference will begin as a fraction of a second, then grow slowly. But over a long enough horizon, midnight on a clock will become less tightly tied to the exact moment when the planet points that longitude away from the Sun.

For centuries, timekeeping tried to make instruments imitate the heavens. Then the instruments became better at counting than the heavens were at turning. The leap second was the uneasy truce: one extra tick, inserted at the edge of the year, to preserve the old promise that noon should still mean something in the sky.

The truce is ending. Future clocks may run more smoothly by admitting that civil time is an agreement, not a mirror. Somewhere ahead, a midnight will arrive with no extra second waiting behind it, and the stars will continue moving anyway.