World Timekeepers Vote to Replace Leap Second with Leap Hour
Delegates from 69 nations approved a shift from the leap second to a leap hour, aiming to reduce technical disruptions caused by irregular minute lengths.
The International Bureau of Weights and Measures announced that delegates from 69 countries voted in Versailles to replace the leap second with a leap hour starting next year. The change is intended to minimise the risk of computer systems and other technical infrastructure malfunctioning when a minute contains 61 or 59 seconds.
"After 30 years of debate, the leap second is making way for the leap hour," said Dr. Setnam Shemar, principal scientist in the time and frequency department at the UK National Physical Laboratory. He added that the new system will preserve the link between Coordinated Universal Time (UTC) and Earth’s rotation while reducing risks to national infrastructure.
Under the new arrangement, UTC may drift up to an hour from astronomical time before a leap hour is required, a scenario that experts estimate is unlikely for thousands of years. Shemar noted that society already tolerates hour-long adjustments for daylight-saving time without major issues.
Leap seconds, introduced in 1972, have been added 27 times to keep UTC aligned with Earth’s variable rotation. Recent observations of a temporary acceleration in Earth’s spin raised the probability of a negative leap second—a subtraction of a second from UTC—within the next decade, prompting the vote.
"Leap seconds can be tricky enough to deal with," Shemar explained, citing incidents in 2012 when some operating systems could not handle a 61-second minute, causing servers to freeze and affecting services such as Reddit, LinkedIn, and airline schedules.
Implementing a leap hour would shift the responsibility for occasional large adjustments to a far-future date, reducing the need for costly, frequent software updates. While the United Kingdom expressed concerns about preserving the connection to Greenwich Mean Time and Russia raised issues regarding its GLONASS navigation system, the overall decision was supported by the IERS Earth Orientation Centre.
"It’s kind of a philosophical discussion, because if a leap hour happens, it won’t be for a very, very long time," said Dr. Christian Bizouard, director of the centre, adding that the vote essentially postpones the decision to a distant future.