The Leap Second Added One Second to Our Clocks. From 2027, It Stops

Earth turns a little unevenly, so a day measured by its rotation slowly drifts from a day counted by atomic clocks. UTC uses the atomic clock's second, but when its difference from time measured by Earth's rotation (UT1) was about to approach 0.9 seconds, one second was inserted to bring them back together. That second is the leap second.

Leap seconds began in 1972 and have been added 27 times in all, every one of them adding a second. The last was on December 31, 2016. That day the UTC clock went from 23:59:59 through 23:59:60 before rolling over to midnight. In Korean time, it was 08:59:60 a.m. on January 1, 2017.

On a day with a leap second, clocks have to pass through an unfamiliar value, 23:59:60. For equipment that depends on precisely matched time, such as telecommunications, power grids, and transportation, such a day was a risk of malfunction. Service providers also handled leap seconds in different ways, so their clocks could disagree.

More recently, as Earth's rotation speed has shifted, a forecast emerged that a negative leap second, removing a second, which has never been done, might become necessary. A March 2025 expert workshop estimated that the chance could reach 30% by 2035.

In 2022 the General Conference decided to widen the allowed difference between UT1 and UTC by 2035, and the 28th General Conference in 2026 set the start date as May 20, 2027. From that day UTC will have no leap seconds, and the two times may drift apart by up to 3,600 seconds, or one hour. The traces of the 27 seconds added so far remain: the difference between International Atomic Time (TAI) and UTC has stayed at 37 seconds since January 2017 and will be kept that way.

The time shown on your watch or phone is likely to change little in everyday use. Where the difference from Earth-rotation time is needed, people will use values published separately, such as navigation satellite signals or international organizations.