No more second jump

This measure may not mean much to the uninitiated, but leap seconds have been a source of controversy in the chronology community for years. Now, they are dead.

Leap seconds have sparked furious debate in the metrology community for three decades. Now, that argument is over: the leap second is officially dead.

It may seem like a novelty to remove an obscure benchmark that the average person was unaware of, but removing a leap second has significant implications for the technology you rely on every day. It also helps prevent what could have been a dangerous moment in time: the possibility of the world's first negative jump in seconds.

The International Bureau of Weights and Measures (known by its French acronym BIPM) announced Thursday that in May 2027, it will stop issuing leap seconds to set the time known as Coordinated Universal Time (UTC), of which 32 have been added since 1972.

The world took a second leap in 1972 due to the growing gap between time measured by atomic clocks and time based on the Earth's rotation. The reason for the difference? The latter is greatly reduced by the tidal friction caused by the Moon and the rotation of the Earth. The Earth's molten core, oceans, and atmosphere also affect the rate of rotation.

Leap seconds are a way of keeping the UTC atomic clock with Universal Time (UT1) astronomically preserved. Consider adding a small extra track for a train that runs exactly parallel to another train. Rather than being done over a fixed time interval—like a leap year—BIPM uses the average of hundreds of atomic clocks around the world and compares its time calculations to astronomical time to determine when a leap second is necessary. The last one was added in 2016.

Of course, adding a song is no small feat, and it might seem like a lot of trouble for a second. But UTC is critical to a variety of technologies given the number of APIs and databases that rely on it. The introduction of a leap second helped keep UTC and UT1 in line, but it also disrupted the various systems that ran in UTC, forcing companies to figure out how to deal with them when they were added every few years.

A number of tech companies, such as Google and Meta, use a "spotting" method to deal with leap seconds. Basically, they spread out a jump over a day or so in small steps. But leap seconds have destroyed other systems. For example, Qantas Airlines stopped its reservation system in one second in 2012, wrote Judah Levin, a fellow at the US National Institute of Standards and Technology, in an email.

Adding a leap second is hard enough. But the possibility of adding a negative leap second has increased somewhat as the Earth has accelerated its rotation in recent years due to climate change. (There's always a climate angle.) According to a 2024 study in Nature , melting ice caps have changed the planet's gravity so drastically that a negative leap second may be necessary in 2029. Part of the calculation for removing a leap second in general was that going negative like that wasn't worth the risk.

"There are ongoing problems with positive leap seconds even after 50 years, and the concern was that a negative leap second would be much more serious because the effects have never been tested," says Levine.

BIPM Time Division Director Patrizia Tavla confirmed these concerns in an email to WIRED. "The possibility of a negative leap second convinced us to set the application date at 2027," he wrote.

Tavla says the group is waiting until May 20, 2027 to implement it because it is the anniversary of the signing of the Meter Convention in 1875, the creation of the BIPM, and also Metrology Day. In other words, it's the perfect time to show the passage of time—or, more precisely, the lack of it.

The death of a leap second makes life easier for everyone. "The good fact is that we have nothing to do!" Tavla says. "Just keep adding up the 'normal' 60 second seconds and minutes."

Levine agrees, writing that "life would be one less hassle."

But kicking the can by adding leap seconds doesn't mean atomic time and astronomical time won't continue to diverge. For this reason BIPM also agreed to establish a leap hour. Given how many leap seconds it takes to add up, a leap hour is thousands of years away. (By the way, there are 3,600 seconds in an hour.)

"We're not talking about leap hour yet," says Tavla. "If the systems are in UTC, they have nothing to do. "If they rely on UT1, they should be able to use UT1-UTC information that will be larger than 1 second [in the future]."