The leap second is over, and the world's clocks may now drift up to an hour from the sky

The world’s timekeepers have decided to stop adding leap seconds: Resolution 5 of the 28th General Conference on Weights and Measures says “continuous UTC will become effective on 20 May 2027,” and it lets the gap between atomic time and the Earth’s actual spin grow to as much as one hour. If you’ve ever had a server choke on a minute with 61 seconds in it, that’s big.

The leap second has been a headache since the International Bureau of Weights and Measures (BIPM) introduced it in 1972. Adding a second to the clock has caused outages at Meta, Reddit and Cloudflare, grounded planes and affected high-frequency stock trading, Scientific American reported. Different organizations handle it differently (Google, for one, smears the extra second across the 24 hours around midnight UTC), and Elizabeth Donley of NIST told Nature in 2022 that this leaves time sources disagreeing by as much as half a second.

The bigger worry was a second going the other way. Scientific American reported that Earth’s rotation started speeding up in 2016, and the resolution cites an estimate that the odds of a negative leap second reach 30% by 2035. The resolution says preparing for one would cost industries about what they spent on the millennium bug. Patrizia Tavella, director of the BIPM’s Time Department, told Scientific American that when they asked users about that 30% risk, “they said, ‘No, even 10 percent risk is too much.’”

Most people will feel little effect, Felicitas Arias, who once ran that department, told Nature. The resolution itself points out that daylight saving time already shifts clocks by an hour in many countries. If you run servers or anything that timestamps across systems, I’d find out now whether your setup smears or steps a leap second, because that code has a retirement date of 20 May 2027.

The clocks stop waiting for the planet next May, and the resolution says that hour of slack keeps UTC running for several centuries.