The Leap Second Is No More

Leap seconds have driven a raging debate in the metrological community for three decades. Now, that argument has been put to rest: The leap second is officially dead.

Doing away with an obscure metric the average person has been blissfully unaware of may seem like a novelty, but killing the leap second has major ramifications for technology you rely on every day. It also helps stave off what could’ve been a dangerous moment in time: The possibility of the world’s first negative leap second.

The International Bureau of Weights and Measurements (known by its French acronym BIPM) on Thursday said that in May 2027, it would stop issuing leap seconds to adjust what’s known as Coordinated Universal Time (UTC), 32 of which have been added since 1972.

The world implemented the leap second in 1972 due to a growing gap between time measured with atomic clocks and time based on the Earth’s rotation. The reason for the discrepancy? The latter is slowing down due in large part to tidal friction caused by the moon and Earth’s rotation not lining up. The molten core of the Earth as well as oceans and the atmosphere also affect the spin rate.

Leap seconds are a way to keep atomic-clock-driven UTC in line with astronomically kept Universal Time (UT1). Think of it like adding a little extra track for a train to keep running exactly parallel with another one. Rather than happening on a set interval—like a leap year—BIPM uses the average of hundreds of atomic clocks around the world and compares their time calculations to astronomical time to determine when a leap second is necessary. The most recent one was added in 2016.

Of course, adding track is no small effort, and it may seem like a lot of hassle for a measly second. But UTC is vital for all sorts of technology given the number of APIs and databases that rely on it. Introducing a leap second helped keep UTC and UT1 aligned, but it also wrought havoc on various systems running on UTC and forced companies to figure out how to deal with them when they were added every few years.

A number of tech companies like Google and Meta use “a ‘smear’ method of dealing with leap seconds,” Judah Levine, a fellow at the US National Institute of Standards and Technology, writes in an email. In essence, they spread a leap second out over the course of a day or so in tiny increments. But leap seconds have thrown off other systems; the airline Qantas, for example, saw its booking system borked by a leap second in 2012.

Adding a leap second is hard enough as it is. But the odds of having to add a negative leap second have been rising in part because the Earth has picked up rotational speed in recent years due to climate change. (There’s always a climate angle.) Melting ice has caused a shift in the planet’s gravity so extreme that a negative leap second may have been required as soon as 2029, according to a 2024 Nature study. Part of the calculus of ditching the leap second altogether was that going negative isn’t worth the risk.