After decades of discussion, objections and concerns about clocks in computer systems, the world’s official timekeepers voted to replace what many saw as a tedious correction: the leap second. It’s time, they say, for leap hour. Delegates from 69 countries cast their votes Thursday at a conference in France of the International Bureau of Weights and Measures, which ensures the world adheres to the same standards for time, weight, distance and more. The decision to adopt leap time starting next year is intended to reduce the risk of computers and other technical systems stumbling as their software struggles with minutes that last 61 seconds (and, worse, 59 seconds) instead of a round. 60. “After 30 years of debate, leap second is giving way to leap time,” said Dr. Setnam Shemar, senior scientist in the time and frequency department at the UK’s National Physical Laboratory (NPL). “This will reduce risks to national systems and infrastructure that depend on accurate time, while preserving the fundamental link between our global time system and the Earth’s rotation.” The vote at Versailles means that Coordinated Universal Time (UTC) can vary by up to an hour from astronomical time before a leap hour is needed, but that is unlikely to happen for thousands of years. sleep,” said Shemar, who was part of the international committee of timekeeping experts that proposed leap hour. The more familiar leap year was introduced more than 2,000 years ago to ensure that the calendar year did not disappear from the time it takes the Earth to orbit the sun. Adding a day to the end of February every four years or so is sufficient, because the Earth orbits the Sun every 365 and a quarter days. Leap seconds are a much more modern invention. In 1955, Louis Essen and Jack Parry of the NPL built the world’s first practical, high-precision cesium atomic clock. The device revolutionized the measurement of time and in 1967 the astronomical definition of a second, based on the Earth’s rotation, was abandoned. Since then, the formal definition of a second has been 9,192,631,770 vibrations of the cesium atom. Its timing has become critical for modern computing systems, communications, satellites, business systems and power grids. But the Earth’s rotation is much less reliable. Its spin is affected by the planet’s molten core, changing oceans, global jet streams and other natural forces. Earth is slowing down: When an asteroid doomed the dinosaurs, a day lasted about 23 and a half hours. Because of this, officials have added 27 leap seconds since 1972 to ensure UTC is aligned with astronomical time. The strategy lasted more than 50 years until scientists detected a new reason to fear technological chaos. In the last decade, the Earth’s rotation has accelerated, meaning officials may have needed to add the first second. negative leap seconds, equivalent to cutting one second from UTC. The 30% probability of a negative leap second in the next decade advanced the Versailles vote. “Leap seconds can be quite difficult to deal with,” Shemar said. He explained that telecommunications, power grids and navigation systems used high-precision timing, but software updates could apply leap seconds at the wrong time, desynchronizing networks. positive intercalates,” Shemar said. Among the websites that crashed were Reddit and LinkedIn. On the other hand, Qantas flights were delayed. Negative leap seconds carry even greater risks. “There could be software that hasn’t been tested on negative leap seconds because we’ve never had to apply one before,” Shemar said. Testing all the computer systems that communicate with each other around the world would cost companies a fortune. It was this concern that influenced the vote. Historically, the United Kingdom has complained about retaining leap seconds because of the link to Greenwich Mean Time. Russia has also opposed the move, citing concerns about the impact on its Glonass satellite navigation system. The IERS Earth Orientation Center in Paris tracks changes in the Earth’s rotation and other motions that feed UTC. The center also issues warnings when leap seconds must be added. Dr. Christian Bizouard, the center’s director, approved the vote but said he conveyed the decision to people in the distant future. “It’s kind of a philosophical discussion, because if a leap hour happens, it won’t be for a long, long time,” he said. “What we are doing is delaying the decision.”
