Continuous UTC to start on May 20, 2027

The General Conference on Weights and Measures has decided that continuous UTC will take effect on May 20, 2027, ending the leap-second adjustment mechanism.

The change widens the permitted difference between civil time and Earth-rotation time; it does not order an immediate one-hour clock change or alter the length of the day.

Why can one second cause trouble?

Atomic clocks keep a steady beat, while Earth’s rotation varies. Leap seconds have kept the two timescales close since 1972. A time step can nevertheless disrupt systems in which tiny differences determine the sequence of transactions or telecommunications events. Providers also spread adjustments over different intervals, which can leave their systems showing inconsistent times.

What does the 3,600-second limit mean?

Resolution 5 sets the maximum permitted difference between Earth-rotation time, UT1, and UTC at 3,600 seconds: one hour. It allows a difference to accumulate over a very long period rather than creating that gap immediately. It is separate from time zones and daylight saving. Readers do not need to move a British, Korean or other civil clock forward by an hour today.

Has a second ever been removed?

Leap seconds applied so far have added a second. Faster rotation could instead require removing one, but a negative leap second has never been implemented. The resolution cites risks for critical infrastructure unprepared for that operation as a reason for acting earlier. This is a risk assessment, not a prediction that communications or financial systems will fail on a particular date.

What should be checked before implementation?

The October 15 adoption was checked against the BIPM resolution and a specialist outlet’s independent interviews. Some pre-vote proposal wording remains in that article; the adopted resolution governs the confirmed details. Implementation and time-distribution notices from BIPM and IERS are the next reference points. Earth-rotation monitoring continues, while a remote future correction is not an already scheduled one-hour clock shift.

Primary sources and independent checks

Adopted resolution: effective date, 3,600-second limit and negative-leap risk; English version, with French the official text: BIPM — Resolution 5 (2026)

Independent IERS/BIPM interviews and adoption update; tool-accessed original under direct HTTP restriction; adopted resolution overrides residual proposal wording: Timeanddate — Graham Jones, updated 15 October 2026

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