Leap seconds policy could end to avoid future timing bugs

The vote replaces leap seconds with a far rarer leap hour. Instead of keeping atomic time within about a second of the time set by Earth’s spin, timekeepers will now let the two drift apart until the gap reaches an hour, or 3,600 seconds, and only then correct it.

The decision came at the 28th General Conference on Weights and Measures (CGPM), the meeting held every four years by the International Bureau of Weights and Measures (BIPM), which runs UTC. The resolution passed with a majority, not unanimously, and country-by-country tallies had not been published. Scientists suspect Russia voted against, since its GLONASS satellite navigation system, unlike GPS or Europe’s Galileo, was built to include leap seconds.

Noël Dimarcq, the French scientist who put the resolution forward, called it a historic moment for UTC. Patrizia Tavella, who heads the BIPM’s time department, summed up the upside in a line many engineers will like: “we have nothing to do.”

Atomic clocks and the planet don’t keep the same beat. Since 1967, a second has been defined by the steady vibration of cesium-133 atoms, and a standard day is exactly 86,400 of those seconds. Earth’s actual rotation wobbles, though. Over thousands of years, the Moon’s pull on the oceans has created tidal friction that slowly stretches the day.

The rotation-based time scale, called UT1, therefore drifts away from atomic time. Starting in 1972, timekeepers added a leap second whenever that gap threatened to pass 0.9 seconds. The last one went in at the end of 2016, and every single one so far has been an extra second, never a missing one.

Here’s the twist that sped things up. From the mid-2010s, Earth started spinning a little faster, not slower. If that continued, the world might need its first negative leap second, a moment when clocks skip a second instead of adding one. Some forecasts put that as early as 2029.

Engineers compare that to a “mini Y2K.” Software has had decades to cope, imperfectly, with an extra second. Almost nothing has ever been tested against time jumping forward without warning. A senior official at the U.S. Naval Observatory, which manages the country’s GPS time, said avoiding negative leap seconds was a big reason to widen the allowed gap.

Countries had already agreed at the 2022 conference to scrap leap seconds by 2035. The negative-second risk pulled that timeline forward to 2027.

For everyday life, almost nothing changes. Minutes still have 60 seconds, days still have 24 hours, and your phone won’t show anything new on May 20, 2027. The sun won’t rise noticeably out of step with your clock for many lifetimes.

The real change is behind the scenes. Many operating systems assume every day lasts exactly 86,400 seconds and that time only ever moves forward. A leap second breaks both rules by creating an 86,401-second day. Big tech firms built workarounds, but different organizations handled the extra second in different ways, which is exactly how outages started.

Without leap seconds, the clock that servers, payment systems, power grids and phone networks share becomes continuous. That means fewer surprise outages tied to time, fewer weird timestamps in logs and less risk for systems that check security certificates or schedule tasks to the second.

Small, hard-to-update gadgets are part of the argument too. Internet-of-things devices like smart plugs, sensors and cameras rarely get patches, so a once-in-a-while time jump is a poor fit for them.

Not everyone gets a free pass. Astronomers and earthquake researchers who care about Earth’s exact position will need to do extra math, because official time will slowly slide away from the planet’s rotation. Navigation systems built around leap seconds, like GLONASS, will need changes.

And the leap hour is a problem handed to the far future. Sources disagree on how far: some scientists say it won’t be needed for more than 1,000 years, while others put it at over 3,000. Either way, nobody alive today is likely to see one.

If you run servers or networks, the practical step is simple. Check how your systems and vendors currently handle leap seconds, and plan for a world after May 2027 where that code path should simply never fire.