Scientists voted to end the leap second and let a clock with seconds follow UTC without constant fixes. The new rule will let UTC drift from Earth’s rotation by as much as 3,600 seconds. The change takes effect on May 20, 2027, after decades of debate.
Noël Dimarcq spoke just before the vote in Versailles, France, and said, “After 30 years of debates, we are ready.” The vote at the General Conference on Weights and Measures passed by a majority. That sets a new limit for the gap between UTC and UT1, which had been kept within 1 second.
UTC and Earth in Versailles
UTC is the reference for how computers, phones, boats, planes, satellites and GPS tell time. It is calculated from a weighted average of atomic clocks around the world. The BIPM is the official source of UTC.
The first leap second was added in 1972. Since then, 27 leap seconds have been added. Those insertions were meant to keep UTC aligned with Earth’s rotation, which does not stay perfectly steady.
Nick Stamatakos on UTC and Earth
Nick Stamatakos, head of the Earth Orientation Department at the U.S. Naval Observatory, said, “Avoiding negative leap seconds is a very big driver to open the window.” Scientists began urgently trying in 2022 to find a practical fix, after Earth’s spin began speeding up in the mid-2010s.
A negative leap second would mean removing a second from UTC instead of adding one. Engineers said that would be like a mini Y2K event. The new hour-long tolerance avoids that kind of adjustment by letting the world’s standard timescale and Earth’s rotation drift much farther apart.
From 1 second to 3,600 seconds
The old rule kept UTC and UT1 within 1 second of each other. The new rule widens that gap to 3,600 seconds. That is 60 minutes, or 3,599 seconds more room than before.
The unresolved question is how often the leap hour will actually be added, because the resolution sets the limit but does not spell out the cadence. For now, timekeepers have chosen a standard that can absorb far more drift before the next correction arrives.
