No more leap seconds

The leap second will be abolished on May 20, 2027. The General Conference on Weights and Measures made the decision Thursday in Paris. There will also be no leap second before May. Instead, leap hours are planned. It will be a long time before a leap hour is needed — more than 1,000 years, at any rate. heise online will report on it.

Leap seconds were introduced in 1972 to synchronize Coordinated Universal Time (UTC), determined precisely by atomic clocks, with Universal Time (UT1), based on Earth’s rotation, which is not measured quite as precisely. When the difference reached 0.9 seconds, a second (23:59:60) was added on June 30 or Dec. 31. This happened 27 times, most recently at the end of 2016.

But the trend has reversed in recent years: since around the start of the decade, an Earth rotation has, on average, taken less — not more — than 86,400 atomic seconds. Over time, this could require a negative leap second, which technical systems aren’t prepared for. In 2025, experts estimated the probability that a negative leap second would be needed by 2035 at 30%.

3,599 seconds is OK

Influential technology operators, in particular, have therefore pushed to abolish leap seconds altogether, whether negative or positive. That’s why the 27th General Conference on Weights and Measures decided in 2022 to discuss with the International Telecommunication Union and other stakeholders how to increase the permitted deviation by 2035 at the latest. No measures should be necessary for at least 100 years; leap minutes were considered in particular.

The consultations led to a more drastic measure: UTC and UT1 will be allowed to differ by as much as a full hour in the future, as determined at the 28th General Conference on Weights and Measures (Resolution 5). Only once the difference reaches 3,600 seconds will a leap hour be used to restore order. Experts estimate that more than 1,000, perhaps even 3,000, years could pass before then.

The case for permanent daylight saving time

A key argument for a theoretical leap hour instead of potentially actual leap minutes is that many countries already have time zones that differ from true solar time by more than an hour. The recent trend toward year-round daylight saving time has made these discrepancies even larger. Earth’s rotational variations matter even less as a result.

In addition, different technical systems have handled leap seconds differently. For example, the clocks in GPS satellites ignore leap seconds and are therefore 18 seconds ahead of UTC. GLONASS, by contrast, incorporates leap seconds directly into its timekeeping.

From now on, programmers won’t have to worry about this anymore. Incorrect leap-second commands will, hopefully, be a thing of the past.

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