The long‑standing debate over leap seconds within the metrology community has finally reached a conclusion: the leap second will no longer be used. While most people have never noticed this obscure adjustment, its removal will have significant effects on everyday technology and will prevent a potentially problematic situation—a negative leap second.
On Thursday, the International Bureau of Weights and Measures (BIPM) announced that it will cease adding leap seconds after May 2027, ending a practice that began in 1972 and added 32 seconds to Coordinated Universal Time (UTC). The leap second was introduced to reconcile the difference between atomic time, which is extremely precise, and Universal Time (UT1), which is based on Earth’s rotation. The planet’s spin is gradually slowing because of tidal friction from the Moon and because the rotation does not perfectly align with the lunar orbit. Variations in the Earth’s molten core, oceans, and atmosphere also influence the rotation rate.
Leap seconds keep UTC, which is driven by atomic clocks, in step with UT1, the astronomical measure of time. The process works like adding a short stretch of track to keep one train aligned with another. Rather than occurring on a fixed schedule, BIPM examines the average of hundreds of atomic clocks worldwide and compares the result to UT1 to decide when a leap second is needed. The most recent insertion took place in 2016.
Implementing a leap second is not a trivial task, and it can cause problems for systems that depend on precise timestamps. Many APIs and databases rely on UTC, and the occasional extra second has forced engineers to devise workarounds. Companies such as Google and Meta use a “smear” technique, spreading the extra second over several hours in tiny increments, according to Judah Levine of the U.S. National Institute of Standards and Technology. Other systems have suffered more dramatically; for example, Qantas experienced a booking‑system failure after the 2012 leap second.
The prospect of a negative leap second has become more likely because Earth’s rotation has recently accelerated, a change linked to climate‑related shifts such as melting ice altering the planet’s mass distribution. A 2024 study in Nature suggested that a negative leap second might have been required as early as 2029. The risk of an untested negative adjustment contributed to the decision to abandon leap seconds altogether. “There are continuing problems with positive leap seconds after 50 years, and the concern was that the negative leap second would be much more serious because its effects had never been tested,” Levine said.
Patrizia Tavella, director of BIPM’s time department, echoed those concerns in an email to WIRED. She explained that the chance of a negative leap second helped set the 2027 deadline, and that recent industry‑standards testing had shown multiple failures when a negative leap second was simulated. The chosen date, May 20, 2027, coincides with the anniversary of the 1875 signing of the Metre Convention, which created BIPM, as well as Metrology Day—a symbolic moment to mark the end of an era in timekeeping.
The removal of leap seconds simplifies matters for most users. “The nice fact is that we have nothing to do,” Tavella said. “Just continue to accumulate seconds and normal minutes of 60 seconds.” Levine agreed, noting that “life will have one less hassle.”
Eliminating leap seconds does not stop the gradual drift between atomic time and astronomical time. To address the long‑term divergence, BIPM plans to consider a “leap hour” in the distant future, although such an adjustment would not be needed for many millennia. “We do not speak of a leap hour yet,” Tavella explained. “If systems rely on UTC, they have nothing to do. If they rely on UT1, they must be able to use the information UT1‑UTC, which in the future will be larger than one second.” The responsibility for handling that gap will fall to future generations.