The leap second will be retired on May 20, 2027, eight years earlier than the 2035 date previously planned.
The International Bureau of Weights and Measures (BIPM), which maintains the International System of Units, voted to move the date up. A leap second is an occasional extra tick added to Coordinated Universal Time (UTC), the global civil time standard, to keep it aligned with Earth's rotation. That rotation is not steady. The atmosphere, oceans, core and even the Moon all nudge it. The BIPM said there is a 30% probability that a "negative leap second" will be needed by 2035. The last positive one pushed clocks to 23:59:60 on December 31, 2016, before they rolled over to 00:00:00 on January 1, 2017.
A negative leap second would mean skipping a second, and that has never been done. The BIPM said it "is considered to pose a high risk of causing anomalies and disruption to critical infrastructures that are largely unprepared." It estimated the preparation cost as similar to the millennium bug effort. Unlike leap years, leap seconds arrive on no fixed schedule, so programmers cannot code them in ahead of time.
The vote reads as a cost comparison. Reddit crashed after the 2012 leap second, and that was the familiar direction, an added second. Nobody has had to handle the reverse in live systems. A desktop PC syncing to an updated time server will probably shrug it off, but the BIPM is worried about transportation, energy distribution, telecommunications and space projects. Rather than spend on software updates and replacement hardware over the next 10 years, it chose to drop the mechanism.
The fix is a leap hour, a one-hour correction in the distant future. The BIPM says it would keep UTC in sync with atomic time (TAI) for several centuries, and it points to time zones and daylight-saving changes as proof that people already live with hour-sized shifts. That is true for people. Whether the infrastructure running under them handles a one-hour jump any better than a one-second skip is a question nobody has to answer yet.