The Earth has been spinning unusually fast recently. Last year on July 4, our planet set a record by completing a full spin 1.66 milliseconds (0.00166 seconds) faster than usual, according to timeanddate.com. One year later, on July 10, 2025, Earth completed a daily rotation that scientists estimate was 1.36 milliseconds faster than usual, giving us another particularly short day. Other shorter (but ever-so-slightly longer) days occurred on July 9 and July 22, although the exact margins have yet to be confirmed.
Losing a couple milliseconds may seem insignificant to most of us—perhaps justifiably so. But tiny error margins in time can mess up systems that depend on extremely precise calculations, such as high-speed communication networks, GPS, or banking systems. As such, scientific timekeepers use highly sophisticated atomic clocks to set the standard via the Coordinated Universal Time (UTC). But with the recent acceleration in Earth’s rotation, the need for a “negative” leap second has re-emerged among some timekeeping experts.
Scientists regularly apply a leap second to keep UTC synchronized with astronomical time, which they base on Earth’s rotation. A full day on Earth—the time it takes our planet to complete one full rotation on its axis—lasts for 86,400 seconds. But factors such as the Sun’s position, the Moon’s orbit, and Earth’s gravitational field influence how quickly the Earth completes its daily cycle. As a result, Earth’s rotation ends up being irregular, and slight differences between UTC and astronomical time can add up in the long run, causing a mismatch between the two.
Leap seconds correct for this deviation. By the same logic, a negative leap second would subtract an extra second from UTC to account for the milliseconds we’re losing from Earth’s faster rotation. Now, this may seem perfectly reasonable, but not all scientists agree. In fact, some scientists found the leap second so problematic that, in 2020, an international group of experts voted to phase out the practice by 2035.
As computing networks became more globally interconnected, the leap second began to cause “failures and anomalies in computing systems,” Patrizia Tavella, director of the International Bureau of Weights and Measures’ time department, told Live Science in a 2022 interview. Moreover, countries account for leap seconds in different ways, causing major complications for airlines scheduling international flights, she said.
Critics of the proposed negative leap second cite similar concerns. To be clear, no formal institution or body is currently advocating for the negative leap second. But should that happen, squeezing in the negative leap second to our timekeeping system will be difficult given the increasingly interconnected nature of our society, Darryl Veitch, a computer networking expert, explained to Live Science in a recent interview.
“There are continuing problems with the insertion of positive leap seconds even after 50 years,” Judah Levine, a physicist at the University of Colorado, told Live Science. “And this increases the concerns about the errors and problems of a negative leap second.”
It seems unlikely, therefore, that scientists will actually adopt the negative leap second, especially since they’ve already decided to retire the positive leap second. But given Earth’s recent shorter daily spins, astronomical time might eventually fall behind UTC, forcing the need for negative leap seconds. Levine puts the likelihood of this happening at 30% in the next decade or so, although last year, Duncan Carr Agnew, an oceanographer at the Scripps Institution of Oceanography, argued in a paper from last year that this could occur as early as 2029. However, Veitch also believes our planet might slow down soon, which would be consistent with longer-term trends on record.
But we’ll just have to see—and you can, too! Timekeepers estimate that our next “short” day will fall on August 5.
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