Timekeepers Propose Leap Hour to Replace Leap Second by 2035
International timekeeping authorities are considering a significant shift in how the world maintains Coordinated Universal Time (UTC), with officials proposing the introduction of a “leap hour” to replace the current system of leap seconds. The move comes as global timekeepers at the General Conference on Weights and Measures (CGPM) work to finalize a replacement for the leap second, which was officially voted to be phased out in 2022.
While the original deadline to decide on an alternative was set for 2035, recent fluctuations in the Earth’s rotation have accelerated the need for a new strategy. Timekeepers are scheduled to vote on the leap hour proposal during the next CGPM meeting this October.
The Technical Burden of Precision Timekeeping
For decades, the International Bureau of Weights and Measures (BIPM) has added leap seconds to UTC to align it with UT1, or astronomical time. This practice was introduced in 1972 to account for the discrepancy between high-precision atomic clocks and the Earth’s inconsistent rotation. While a difference of a few milliseconds may appear negligible to the public, it presents critical challenges for the modern, computerized infrastructure that powers communications, banking, and global GPS networks.
The inclusion of leap seconds has frequently caused technical complications. A notable example occurred on January 1, 2017, when a leap-second-related bug caused widespread failures in Cloudflare’s DNS service. Experts, including Judah Levine, a former physicist at the U.S. National Institute of Standards and Technology, have noted that the leap second has long been viewed as a problematic element in global timekeeping, effectively acting as a “technical nightmare” for modern digital systems.
Earth’s Erratic Rotation and the Risk of “Negative” Seconds
Although the long-term trend of Earth’s rotation is a gradual slowing—largely due to tidal effects from the moon—the planet also experiences unpredictable short-term speedups. These variations are influenced by factors ranging from the distribution of mass in the Earth’s molten outer core to shifts in polar ice and atmospheric angular momentum. Even significant seismic events, such as the 2011 earthquake in Japan, have been shown to impact the planet’s rotation.
Since 2016, the Earth has begun to spin unusually fast. Records set in recent years illustrate this trend: on July 4, 2024, the Earth completed its daily rotation 1.66 milliseconds faster than average. This was followed by another rapid rotation on July 10, 2025, which was 1.36 milliseconds faster than usual.
This acceleration has raised the prospect of a “negative” leap second—a requirement to subtract a second from UTC to maintain alignment with astronomical time. According to Patrizia Tavella, director of the time department at the BIPM, the risk of needing a negative leap second is significant. “We estimated that if we wait till 2035, we have 30 percent risk of a negative leap second,” Tavella told *Scientific American*. She added that stakeholders have expressed that even a 10 percent risk is considered unacceptable.
Why a Leap Hour is the Proposed Solution
The CGPM resolution suggests that a leap hour could provide a more stable, manageable alternative to the frequent, smaller adjustments of the leap second. By implementing a larger margin of time, the frequency of necessary adjustments would be drastically reduced, allowing stakeholders and industries more time to prepare for synchronization changes.
The following table summarizes the history and context of leap adjustments:
| Factor | Details |
|---|---|
| Total leap seconds added since 1972 | 27 |
| Last recorded leap second | December 31, 2016 |
| Primary cause of rotation speedup | Mass distribution, atmospheric shifts, and seismic events |
| Primary cause of rotation slowing | Tidal effects (ocean tides) |
| Target date for new timekeeping standard | 2035 |
As the October vote approaches, timekeepers are balancing the necessity of keeping UTC aligned with astronomical time against the reality that our modern electronic world requires a less disruptive approach to time management. Tavella has emphasized that there is “sufficient urgency” to implement a new, more reliable standard as soon as possible to avoid the complications inherent in the existing, volatile system.
Find more reporting in our Tech section.
