Time would have to move backward, breaking every assumption software ever made
For decades, Earth's gradual slowing had been a quiet, manageable fact — one that atomic timekeepers accommodated with occasional leap seconds. Then, in mid-2020, the planet reversed course, spinning faster than it had in half a century, with July 19 marking the shortest day ever recorded. The acceleration, driven by glacier melt, shifting crustal mass, and forces both slow and sudden, poses no existential threat to the planet itself — but it places the entire architecture of global digital time in uncharted territory, where even a single subtracted second could unravel systems built on the assumption that time only moves forward.
- Earth completed its fastest rotation on record on July 19, 2020 — 1.4602 milliseconds shorter than a standard day — with 28 days that month alone breaking records that had stood since 2005.
- The acceleration is being driven by glacier melt redistributing planetary mass and making Earth rounder, compounded by seismic events like the 2011 Japan earthquake that physically reshaped the planet's crust.
- Timekeepers in Paris now face the unprecedented prospect of a negative leap second — not adding time, but subtracting it — something no atomic clock system has ever attempted.
- The last positive leap second in 2015 shut down the New York Stock Exchange for 61 minutes and crashed major platforms including Twitter, Netflix, and Amazon; a negative second would be technically far more disorienting.
- IERS declined to act on December 31, 2020, but the next decision window — June 30, 2021 — looms as engineers and scientists watch to see whether Earth's spin stabilizes or continues its record-breaking pace.
For more than a century, Earth had been slowly winding down, its rotation growing just sluggish enough that atomic timekeepers periodically inserted a "leap second" to keep their clocks aligned with the planet's actual position in space. Twenty-seven such seconds had been added since the 1970s. Then, in the summer of 2020, Earth began spinning faster.
By July, the shift was undeniable. On July 19, Earth completed a full rotation 1.4602 milliseconds ahead of schedule — the shortest day ever recorded — and 28 days that month broke records that had stood since 2005. The International Earth Rotation and Reference Systems Service, the Paris-based body responsible for global timekeeping, found itself weighing something it had never done before: a negative leap second, subtracting time rather than adding it.
The causes are layered. Climate-driven glacier melt redistributes mass across the planet, making Earth slightly rounder — and like a figure skater drawing in their arms, a rounder Earth spins faster. Catastrophic events contribute too: the 2011 magnitude-9.0 earthquake off Japan physically shifted the crust and shaved 1.6 millionths of a second off that day's length.
For scientists, the acceleration is a curiosity. For the technology world, it is a potential crisis. Modern digital infrastructure depends on atomic time staying synchronized with solar time to the second. When a positive leap second was added in June 2015, the New York Stock Exchange shut down for 61 minutes, and Twitter, Netflix, Amazon, and Apple all experienced outages. A negative leap second would be worse — instead of adding a second, it would force time to move backward, a scenario programmers have never had to engineer for. In Unix-based systems, a second would simply cease to exist in the record.
IERS chose not to act on December 31, 2020, deferring the question. But the next window arrives June 30, 2021, and whether Earth's spin continues to accelerate, levels off, or reverses remains unknown — leaving the global timekeeping system, and everything that depends on it, waiting.
For more than a century, Earth had been gradually slowing down. Its rotation was becoming sluggish, predictable—a fact that atomic clocks, those instruments of absolute precision, had to account for by occasionally inserting an extra second into the global timekeeping system. Since the 1970s, timekeepers had added 27 of these "leap seconds" to keep atomic time synchronized with solar time, the actual position of Earth relative to the Sun and stars. Then, in the middle of 2020, something unexpected happened. Earth began to spin faster.
By July, the acceleration had become undeniable. On July 19, 2020, Earth completed a full rotation in 1.4602 milliseconds less than the standard 24 hours—the shortest day ever recorded. That month alone, 28 days shattered the previous record for brevity, each one surpassing a benchmark that had stood since 2005. The International Earth Rotation and Reference Systems Service, the Paris-based organization responsible for keeping global time aligned with planetary motion, faced an unprecedented decision: whether to introduce a negative leap second, something that had never been attempted in the history of atomic timekeeping.
The cause of this acceleration remains multifaceted. A 2015 study attributed the speedup to global warming and the melting of glaciers, which redistributes mass across the planet and makes Earth slightly rounder—and a rounder Earth spins faster, much like a figure skater accelerating as they pull their arms inward. But Earth's rotation had also been jolted by more dramatic events. The magnitude-9.0 earthquake that devastated northeastern Japan on April 11, 2011, displaced portions of the Earth's crust and shaved 1.6 millionths of a second off that day's length. The planet, it seemed, was responding to forces both gradual and catastrophic.
For planetary scientists, the acceleration posed no particular alarm. They understood that Earth's spin had always been subject to numerous influences—the gravitational pull of the Moon, variations in snowfall, the erosion of mountains. But for computer scientists and software engineers, the prospect of a negative leap second represented a genuine crisis. Modern technology depends on what they call "true time," a synchronization between atomic clocks and solar time so precise that even a single second's discrepancy can cascade through systems worldwide. When a positive leap second was added on June 30, 2015, the results were chaotic. The Intercontinental Exchange, which operates the New York Stock Exchange, had to shut down for 61 minutes. Twitter, Instagram, Pinterest, Netflix, Amazon, and Apple's music service all experienced outages lasting 40 minutes. Reddit, Foursquare, Yelp, LinkedIn, Gawker, and Qantas had crashed three years earlier when a leap second was inserted in 2012, their servers confused by the expectation that every minute would contain exactly 60 seconds, not 61.
A negative leap second would present an even more vexing problem. Instead of adding time, it would subtract it, forcing time to move backward—a scenario that programmers had never had to engineer for. One software engineer described the nightmare scenario: a Linux system would cycle back through the same second twice, creating a moment that could not be uniquely represented in standard Unix time notation. It would simply vanish from the record. Financial markets, which conduct trades during the day when a leap second adjustment might occur, faced the prospect of genuine havoc. Google had managed the 2015 crisis by "smearing" the extra second across a 24-hour period, gradually absorbing it rather than inserting it all at once. Amazon used a different approach, leaving the two companies out of sync with each other for an entire day.
In July 2020, IERS announced they would not introduce a negative leap second on December 31 of that year, buying time to study the phenomenon further. But their next opportunity to make the adjustment would come on June 30, 2021. Whether Earth's rotation would continue to accelerate, stabilize, or reverse course remained unknown. What was certain was that if the speedup persisted, the global timekeeping system—and the vast digital infrastructure that depends on it—would face a test unlike any in its history.
Notable Quotes
It is certainly correct that the Earth is spinning faster now than at any time in the last 50 years. It's quite possible that a negative leap second will be needed if the Earth's rotation rate increases further, but it's too early to say if this is likely to happen.— Senior research scientist at the UK's National Physical Laboratory
The Hearth Conversation Another angle on the story
So Earth is spinning faster now. Is that something we should actually worry about?
Most of us won't feel it at all. A day being 1.4 milliseconds shorter is imperceptible to human experience. But the systems that run modern life—financial markets, telecommunications, cloud services—they're built on the assumption that atomic time and solar time stay in lockstep. When they don't, things break.
Why did it start spinning faster in 2020 specifically?
That's the puzzle. It wasn't sudden in the way an earthquake is sudden. Glaciers melting from global warming redistribute mass across the planet, making it slightly rounder, which makes it spin faster. But there were also discrete events—like the 2011 Japan earthquake—that physically jolted the rotation. 2020 just happened to be when the acceleration became impossible to ignore.
And they've never had to subtract a second before?
Never. We've added 27 leap seconds since the 1970s. But subtracting one? That's uncharted territory. It means time would have to move backward, which breaks assumptions that every piece of software ever written has made about how time works.
What happened the last time they added a leap second?
In 2015, the New York Stock Exchange shut down for 61 minutes. Twitter, Netflix, Amazon, Apple—all offline for 40 minutes. Smaller outages in 2012 took down Reddit, Yelp, LinkedIn. A single extra second cascaded through systems that weren't built to handle it.
So a negative leap second could be worse?
Potentially much worse. At least with an extra second, you're adding something. Subtracting it means time loops back on itself. One programmer said it would be "an appalling sh**show." That's the technical consensus.