The 28th General Conference on Weights and Measures decided: continuous UTC takes effect on 20 May 2027. Leap seconds, which nudged atomic clocks toward Earth’s rotation, will no longer be inserted. The maximum |UT1−UTC| difference rises to 3,600 seconds — an hour. Metrologists think that buffer will last for centuries.
Why now
Transport, networks, energy and space need a scale without steps. A one-second jump already breaks services; a negative leap second is worse, and it has never been used. A CCTF and IERS workshop in March 2025 put the chance of a negative second by 2035 at 30%. Industry prep is compared to Y2K. Providers already dodge the insertion however they can — and the scales between systems drift apart.
If UTC stays jagged, big players will leave for GNSS time. Then the international standard stops being shared. Radio astronomy and navigation still take UT1−UTC from satellite messages and IERS services: for a telescope an hour of offset is a term in a formula, not a disaster. For a smart meter it is another story.
What changes for people
Civil clocks already live in time zones and daylight saving, where hour shifts are familiar. Cancelling the leap second will not touch an ordinary alarm. It will touch software that waited for an IERS announcement and only knows how to “insert +1”. That stack needs to live without a step by May 2027.
How to look at it
This is a rare case where metrology is directly saving critical infrastructure from a future outage. The price is that UTC will slowly drift from solar days, but not on a human lifetime. NTP admins and exchanges should check the stack before 20 May 2027, not wait for “one more second in December”.
