What is a leap second? And why does Big Tech want to alter time?

Get the context and find out the "why" behind the stories shaping our world


What does it take to bring Big Tech to its knees? A leap second.

What unites Meta, Amazon, Microsoft and the U.S. government? Advocating for the end of “leap seconds” — blink-and-you’ll-miss-it adjustments to timekeeping that compensate for wobbles in the Earth’s rotation.

Scientists have added an extra second 27 times since 1972 to keep atomic clocks in sync with astronomical time. Tech companies hate the practice because it can wreak havoc on precise technological systems that are much better at telling time than humans are — at least until we insert an extra second. Past additions of leap seconds have caused parts of the internet to go down for hours.

On Monday, Facebook’s parent, Meta, kicked its opposition to the leap second into high gear with a blog post calling to abolish the practice, with Amazon and Microsoft also joining the cause. At best, Meta argues, it corrupts data and crashes websites. “Every leap second is a major source of pain for people who manage hardware infrastructures,” wrote Meta engineers Oleg Obleukhov and Ahmad Byagowi.

“I wouldn’t be sad to see leap seconds go away,” said John Graham-Cumming, chief technology officer at Cloudflare, one of the companies that has experienced disruption from the addition of leap seconds.


Amazon and Microsoft did not respond to requests for comment.

Time after time

Atomic clocks, which underlie international time standards, count time by measuring the natural vibration of cesium atoms, said Elizabeth Donley, chief of the Time and Frequency Division of the U.S. government’s National Institute of Standards and Technology.

Before the advent of atomic clocks in the 20th century, time was measured by the length of solar day, an approach that draws from astronomy. But given the slightly irregular axis tilt and rotation of the Earth, there can be variations between atomic time (Coordinated Universal Time or UTC) and astronomical time, known as Universal Time or UT1.

In short, time is weird and arbitrary.

Every once in awhile, those systems are reconciled with a leap second — which only occurs on June 30 or Dec. 31, and is announced beforehand. What it looks like, practically, is the clock turning from 23:59:59 to 23:59:60 before hitting normal midnight, 00:00:00.


Meta’s blog post highlighted one potential twist that could be coming in the future: The negative leap second. It would help compensate for Earth’s faster-than-expected rotation in recent years. Essentially, the world’s clocks would jump from 24:59:58 on the appointed day to 00:00:00 — skipping one second entirely.

“The impact of a negative leap second has never been tested on a large scale; it could have a devastating effect on the software relying on timers or schedulers,” the Meta post argues.

A leap second, whether positive or negative, that does not exist in the programming of computers can cause them to crash. That‘s part of the reason people were so worried about Y2K as the year 2000 approached. At that time, many computer programs denominated years using only the last two digits, meaning that 2000 and 1900 would look like the same year. While that concern was overblown, the same issues around denominating time is what lies behind criticism of leap seconds.

“Over time this became more and more of a problem with digital networks because technology just became so much more dominant in our society and so now over the past 10 years, when they’re added, they’ve caused a lot of failures in various websites and computer systems,” said Donley. “That’s a big motivation to address them.”

Time for a new system?

It’s not hypothetical or hyperbole. A leap second change triggered outages at Reddit in 2012 and downtime at Foursquare, LinkedIn and Yelp over the years. The most recent leap second occurred in 2015.

The addition of leap seconds can cause glitches if it’s not accounted for properly in a software system, Donley said. “You can have disagreement between the actual time of nodes of the network, for example, or different web pages,” she added. “That can cause failures. And given the updated second comes in at midnight UTC, which, that could be in the middle of the day on the West Coast of the U.S. or Hawaii.”

For its part, Meta has started “smearing” leap seconds, either slowing down or speeding up the clock of its Time Appliance, which is the timekeeping infrastructure of the company over a 17-hour period, dividing the additional second across that span to make it more digestible for computers.

“We had an outage in the middle of the night on the first day of the year because of leap seconds,” said Graham-Cumming, whose employer, Cloudflare, helps provide security and accessibility to millions of websites. “Leap seconds tend to get introduced at midnight on January the first and so we had an outage and saw systems failing.”

He was not surprised to see Meta come out against leap seconds, given its large network and massive server farms. But David Finkleman, the former chief technical officer of the U.S. Space Command, said the recent post from Meta is incomplete and tendentious.

“Every scheme to mitigate leap second corrections has its own anomalies — in this case, installing unique software,” he said. And killing the leap second could create big problems for scientists who study space, he said, because their precise measurements of stars, galaxies and other objects depend on aligning human time with astronomical time to point their telescopes at the right targets.


“Correction for Earth rotation is very important, and smearing is also not synchronous with Earth rotation,” Finkleman said. “Those who despise the leap second have not learned to accommodate it correctly.”

Donley said that while there is significant agreement among governments about eliminating leap seconds, with the notable exception of Russia, no change is imminent. Getting rid of the practice would require various internet and time standards bodies to reach an agreement on doing so and on what would come next.

“It’s pretty much almost everybody,” said Donley. “The way that it all works is by consensus. We make baby steps to try to stop the procedure. It’s been under discussion over and over and over and over again for probably nearly as long as [leap seconds] have been in existence.”

That takes, well, time.

The Consultative Committee for Time and Frequency, a global timekeeping standards body, is expected to vote in November on a resolution to halt the use of leap seconds by or before 2035.


“It’s not clear how many, if any, leap seconds will need to be inserted before then anyway — hopefully none,” said Donley. “Because the rate of insertion of them has dropped and the difference between UTC and the new UT1 is heading toward zero right now.”

Thanks to Alicia Benjamin for copy editing this article.

  • Benjamin Powers
    Benjamin Powers

    Technology Reporter

    Benjamin Powers is a technology reporter for Grid where he explores the interconnection of technology and privacy within major stories.