What is a leap second? And why does Big Tech want to change times?

What unites Meta, Amazon, Microsoft and the US government? Advocating an end to “leap seconds”—a blink-and-you’ll-miss-it adjustment to timekeeping that compensates for wobble in 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 this practice because it can wreak havoc on precise tech systems that are much better at telling time than humans – at least until we put in an extra second. The last addition of leap seconds has caused some parts of the Internet to shut down for hours.

On Monday, Facebook’s parent, Meta, kicked its opposition to Leap Second into high gear with a blog post calling for an end to the practice, with Amazon and Microsoft joining in as well. At best, argues Meta, 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 Oblukhov and Ahmed Bygovi.

“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 adding leap seconds.

Amazon and Microsoft did not respond to requests for comment.


Elizabeth Donnelly, head of the US government’s National Institute of Standards and Technology’s Time and Frequency Division, said atomic clocks, which fall under international time standards, calculate time by measuring the natural vibrations of cesium atoms.

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

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In short, timing is strange and arbitrary.

Every once in a while, those systems are reconciled with a leap second—which only happens on June 30th or December 31st, and is announced in advance. What it looks like, practically, is the clock from 23:59:59 to 23:59:60 before hitting normal midnight, 00:00:00.

Meta’s blog post highlighted a potential turning point that could come in the future: the negative leap second. This will help compensate for the faster-than-expected Earth’s rotation in recent years. Essentially, the world clocks will jump from 24:59:58 to 00:00:00 on the appointed day – skipping a second altogether.

“The effect of a negative leap second has never been extensively tested; it can have disastrous effects on software relying on timers or schedulers,” argues the meta post.

A leap second, whether positive or negative, that is not present in a computer’s programming can cause them to crash. This is why people were so concerned about Y2K as the year 2000 approached. At the time, many computer programs represented years using only the last two digits, meaning that 2000 and 1900 would look like the same year. While that concern was exaggerated, it is the same issue about timing that lies behind the Leap Second criticism.

“Over time this became a problem with digital networks because technology has become so dominant in our society and so now in the last 10 years, when they have been added, they have caused a lot of failures in different areas. website and computer systems,” Donnelly said. “That’s a big motivation for them to address.”

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Time for a new system?

This is not fiction or exaggeration. A leap second change triggered years of downtime on Reddit in 2012 and at Foursquare, LinkedIn and Yelp. The most recent leap second occurred in 2015.

Donnelly said that other than leap seconds, if it is not properly accounted for in the software system, it can cause errors. “You can have real-time access to nodes of the network, for example, or disagree between different web pages,” she said. “It can cause failures. And the updated second one comes at midnight UTC, which could be in the middle of the day on the west coast of the US or Hawaii.

For its part, Meta has begun “smearing” leap seconds, either slowing or speeding up the clock on its Time Appliance, which is the company’s timekeeping infrastructure, over a 17-hour period, in that period. Splits the extra second. It is more digestible for the computer.

“We had an outage on the first day of the year due to leap seconds in the middle of the night,” said Graham-Cumming, whose employer, Cloudflare, helps provide security and accessibility to millions of websites. “Leap seconds are introduced at midnight on January 1st and so we had an outage and the system failed.”

Given their large network and huge server farms, they weren’t surprised to see Meta coming up against Leap Seconds. But David Finkelman, the former chief technical officer of US Space Command, said recent posts on META are incomplete and trending.

“Every plan to mitigate leap second improvements has its inconsistencies — in this case, installing unique software,” he said. And hitting the leap second could pose major problems for scientists studying space, he said, because their accurate measurement of stars, galaxies and other objects depends on aligning human time with astronomical time to ensure that their telescopes to be pointed at the correct target.

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“The correction is very important for the Earth’s rotation, and the smearing is also not synchronous with the Earth’s rotation,” Finkelman said. “Those who hate the second jump haven’t learned to adjust it correctly.”

Donnelly said there is significant consensus among governments about eliminating leap seconds, with the notable exception of Russia, but no change is imminent. Getting rid of this practice would require the various Internet and time standards bodies to do so and reach an agreement on what would happen next.

“It’s too much for almost everyone,” Donnelly said. “The way it all works is by consensus. We make baby steps to try to stop the process. It’s probably been discussed over and over again for almost too long.” [leap seconds] have been in existence.”

This, well, takes time.

The Advisory Committee for Time and Frequency, a global timekeeping standards body, is expected to vote in November on a proposal to stop 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 this — hopefully none,” Donnelly said. “Because the rate of insertion of them has dropped and the difference between UTC and the new UT1 is just going to zero.”

Thanks to Alicia Benjamin for copy editing this article.

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