Delegates in Versailles vote to end the leap second from May 2027

UTC is a weighted average of atomic clocks around the world, disseminated by the International Bureau of Weights and Measures ^[12]. Earth's rotation is irregular, so a UTC day is close to a UT1 day but not identical to it ^[10]. The leap second kept the two within one second of each other ^[10]. From May 20, 2027, the allowed gap is an hour ^[1][3]. That is 3,600 times the old tolerance ^[15].

For operators, the old problem was the step itself. UTC is the reference for computers, phones, boats, planes, satellites and GPS ^[12]. Scientific American reports that switching to continuous time will be easier for digital infrastructure, with much less risk of bugs or service outages from a leap second discontinuity ^[9].

The negative case was the sharper worry. Engineers say a negative leap second would be like a "mini Y2K" because it would suddenly send the entire world one second backward in time ^[7]. The earliest forecast for one, 2029, falls two years after the new effective date ^[16]. "Avoiding negative leap seconds is a very big driver to open the window," said Nick Stamatakos, who runs the Earth Orientation Department at the U.S. Naval Observatory, the agency in charge of the nation's GPS grid ^[11][14].

The leap hour is the backstop, and scientists predict it will not be needed for more than 1,000 years ^[4]. We think no system in service today needs to be designed around a leap hour, provided that forecast holds. The date to plan around is 2027.

The losers from the wider gap are users of the sky. The leap second was first added at the behest of astronomers and other experts ^[5], and Scientific American does not list which systems rely on the one-second bound or how users who need Earth's rotation will get it once UTC stops tracking it.

"After 30 years of debates, we are ready," said Noel Dimarcq, a senior scientist at the Centre National de la Recherche Scientifique, just before the vote ^[8].