World's timekeepers vote to ditch leap second, replace with future 'leap hour'

Twenty-seven times in the past century, our clocks have stopped for a single second. It sounds like a glitch, a hiccup in the machinery of time itself—but it was deliberate, a correction made to keep the ultra-precise atomic clocks that run our world aligned with something far less reliable: Earth's actual rotation.

The problem is ancient but only recently became urgent. By the traditional definition, a second is one eighty-six-thousand-four-hundredth of a day. But Earth does not spin at a perfectly steady rate. Our planet's rotation varies by as much as a millisecond from day to day, slowed over millennia by the Moon's gravitational pull and, more recently, affected by atmospheric winds, ocean currents, melting glaciers, volcanic activity, and shifts in the planet's interior mass. Since around 2020, Earth has actually been spinning faster than it was before—a reversal that caught timekeepers' attention.

In the late 1960s, the International Bureau of Weights and Measures adopted a far more precise definition of a second, based on the oscillations of caesium atoms. This atomic second became the foundation of Coordinated Universal Time, or UTC, the standard that synchronizes GPS, financial transactions, telecommunications, and virtually every digital system on Earth. The atomic clocks that measure UTC are accurate to fractions of a nanosecond. Earth's rotation, by comparison, is sloppy. So beginning in 1972, timekeepers began inserting leap seconds—pausing time for exactly one second—whenever Earth's rotation drifted too far from atomic time. The last one came at the end of 2016.

But leap seconds have become a problem in an age when software assumes time only moves forward and that there are always exactly sixty seconds in a minute. When a leap second arrives, systems crash. Reddit went down. Cloudflare went down. Countless smaller services failed silently. The irregularity made things worse: leap seconds were announced only six months in advance, giving software engineers little time to prepare, and they happened at unpredictable intervals. Unlike daylight saving time or time zones, there was no pattern to anticipate.

The crisis deepened when Earth's acceleration meant the next leap second might need to be negative—time jumping forward instead of backward. This has never happened before. No software has ever been tested for it. In 2022, the BIPM's governing body voted to abandon leap seconds by 2035, but Earth's unexpected speedup forced them to accelerate the timeline. This week, delegates meeting near Versailles will vote on whether to end the practice entirely.

The leading proposal is to let UTC drift from Earth's rotation until the gap reaches a full hour, then insert a "leap hour" to resynchronize. At the current rate of drift—less than a second per year—this would not be necessary for more than a thousand years. By then, timekeeping systems will likely be unrecognizable. Software that crashes today over a single second will probably be automated and self-correcting. The philosophical objection remains: we will be counting time by a standard disconnected from our actual experience of day and night. But for the engineers and scientists who maintain the systems that keep the world synchronized, the practical relief is clear. No more emergency calls on New Year's Day. No more software failures. No more leap seconds.