For half a century, humanity has been adding leap seconds to keep atomic clocks in step with our slowing planet. This summer the script flipped. Earth is suddenly spinning faster, shaving milliseconds off every 24 hours. If the trend holds, the guardians of global time will do something unprecedented: delete a second from Coordinated Universal Time (UTC). Welcome to the era of the “negative leap second.”
The Discovery
At midnight on 29 June 2022, Earth completed a rotation in 1.59 milliseconds less than 86,400 seconds—the shortest day since precise atomic clocks debuted in the 1960s. The record fell again on 26 July 2022 and has been approached on several days in 2023–2024. Data from the Paris-based International Earth Rotation and Space Systems Service (IERS) show that the planet has been running ahead of schedule almost continuously since 2020.
Why the Hurry?
- Core Dynamics: Earth’s liquid outer core can exchange angular momentum with the solid mantle, nudging rotation rate like a spinning skater pulling in her arms.
- Melting Ice Sheets: As glaciers in Greenland and Antarctica shed trillions of tonnes of water, mass migrates from the poles toward the equator, shrinking the planet’s waistline and speeding its spin (conservation of angular momentum).
- Ocean-Climate Feedbacks: Shifts in major ocean currents—partly driven by warming seas—redistribute mass and can either brake or accelerate rotation on time-scales of years to decades.
“The aggregate effect is subtle—milliseconds per day—but unmistakable,” explains Professor Eric Stempels, astronomer at Uppsala University. “For the first time in the atomic-clock age, Earth is consistently ahead of schedule.”
From Leap Seconds to Leap-Second Surgery
Since 1972 we’ve inserted 27 leap seconds to keep UTC within 0.9 s of Earth’s rotation. If today’s faster spin persists, the IERS will post a “negative leap second,” requiring 23:59:58 UTC to be followed immediately by 00:00:00 UTC. That single skipped second could occur as early as 2028.
The Technical Shockwave
Deleting a second is far harder than adding one. Software, trading platforms, telecom networks, and power grids expect time to flow monotonically. When the leap second was introduced, no one anticipated subtraction. “We’re stress-testing patches now,” says Patrizia Tavella, director of the BIPM Time Department. Google and Meta have proposed “smearing” the lost second across an hour or day to blunt the impact, but no consensus exists.
What Comes Next
- Monitoring: Superconducting gravimeters and ring-laser gyroscopes will watch for changes in core-mantle coupling.
- Policy: The November 2026 meeting of the International Telecommunication Union may vote to abolish leap seconds entirely, letting civil time drift from Earth until a larger correction—perhaps a “leap minute” in 2200—is needed.
- Daily Life: Don’t reset your watch just yet. The adjustment will be invisible to most people but critical to the infrastructure that runs the modern world.
Bottom Line
Time, once the most reliable constant, is now a moving target. As the planet pirouettes faster, humanity faces a paradox: we can measure time with staggering precision, but we can no longer take its length for granted.