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In 2022, the international organizations responsible for the accuracy of time measurement decided to phase out the use of leap seconds – extra fractions of time that were periodically added to synchronize clocks with the Earth’s uneven rotation around its axis. However, because the Earth’s rotation speed has been increasing at an unusual rate recently, experts have begun seriously discussing the introduction of a much more extensive correction mechanism: the leap hour.
Every four years, scientists and relevant government agencies gather at the General Conference on Weights and Measures (CGPM) to discuss current developments and approve standards to ensure the accuracy of Coordinated Universal Time (UTC). After the elimination of leap seconds in 2022, conference participants planned to decide on an alternative method with a longer time frame, lasting until 2035. However, recent observations of the planet’s accelerating rotation have forced experts to confront the threat of so-called negative leap seconds. The potential consequences of this phenomenon are prompting the relevant authorities to find a commercial and technical solution well before the previously set deadline of 2035.
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One of the most realistic options for solving this problem is considered to be the complete replacement of the leap second with a leap hour. An official vote on this matter among authorized representatives will take place during the next session of the CGPM, scheduled for October of this year. As Patricia Tavella, director of the Time Department at the International Bureau of Weights and Measures (BIPM), in an interview with Scientific American, stated that according to their calculations, if a decision on this matter is postponed until 2035, the probability of having to introduce a negative leap second will reach 30%.
This urgency is driven by important factors. A full astronomical day on Earth – that is, the duration of one rotation of the planet around its axis – lasts approximately 86,400 seconds. However, celestial phenomena such as the Sun’s current position or the Moon’s orbital motion can cause slight fluctuations in this duration. Thanks to significant advances in scientific instrumentation, modern humanity is now able to accurately record these changes. To eliminate minimal discrepancies between atomic time (UTC) and the astronomical standard (UT1), experts at the BIPM introduced the practice of adding a leap second in 1972.
For most ordinary people, an error of a few milliseconds seems completely insignificant. However, for specialists who maintain high-precision technological systems in the fields of telecommunications, GPS satellite navigation, or banking transactions, even the slightest deviations can completely disrupt the operation of entire digital networks. For example, a specific software bug related to the addition of a leap second to UTC at midnight on January 1, 2017, led to large-scale operational disruptions in Cloudflare’s Domain Name System (DNS). In other words, an ironic situation arose: leap seconds were created precisely to eliminate time discrepancies, but instead turned into a real technical nightmare that completely contradicts the needs of today’s digital society. This was discussed in an analytical piece by The New York Times dedicated to the 2022 resolution.
Some experts in the field agree that the extra second has always posed challenges for the industry. Judah Levine, who previously worked as a physicist in the Time Division at the U.S. National Institute of Standards and Technology, noted this in a comment for Scientific American. However, the problem became even more acute when, starting in 2016, the Earth began rotating abnormally fast.
Specifically, on July 4, 2024, the planet set a new record, completing its daily rotation 1.66 milliseconds faster than usual. Roughly a year later, on July 10, 2025, this process occurred 1.36 milliseconds faster, with other days shorter than normal also recorded on July 9 and 22. This trend has prompted analysts to warn that the introduction of a “negative” second – that is, subtracting one second from the UTC standard to synchronize it with UT1 – will be unavoidable as early as 2029.
Given the large number of technical errors that have occurred during the addition of standard leap seconds, negative adjustments will certainly be no less problematic for software, as was explicitly noted in the text of the 2022 CGPM resolution. In contrast, switching to a significantly longer interval of one hour will substantially reduce the burden on global computing systems. According to CGPM calculations, such large-scale changes will occur extremely rarely, giving all stakeholders and engineers sufficient time to prepare thoroughly.
Patricia Tavella confirmed to reporters that she considers the current situation to be urgent enough to warrant the immediate implementation of a leap hour. She noted that during consultations with end users of the systems, business partners, and other international institutions, they were asked whether they were prepared to accept a 30 percent probability of a sudden leap second occurring. In response, representatives of these organizations unanimously stated that even a 10 percent probability of such an occurrence is an absolutely unacceptable risk for them.
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