Leap seconds end as UTC becomes continuous on 20 May 2027

Leap seconds end as UTC becomes continuous on 20 May 2027

15 October 2026: The General Conference on Weights and Measures has decided that Coordinated Universal Time will become continuous on 20 May 2027, ending the leap seconds that have kept it within a second of the Earth's rotation since 1972. Resolution 5 of its 28th meeting, held in Versailles from 13 to 15 October, allows UT1 and UTC to drift up to 3,600 seconds apart, which the conference says keeps UTC continuous for several centuries. The 2022 conference had set 2035 as the deadline; this one brought it forward by eight years.

Not yet independently verified. The resolution sets the date; no operating system, NTP or cloud provider has yet published how it will apply it. The BIPM page carries no date of its own; we date it to the vote reported by heise. We will update this when it can be confirmed, and remove this note.

Why the date moved

The resolution gives the reason plainly. A workshop of the Consultative Committee for Time and Frequency and the International Earth Rotation and Reference Systems Service in March 2025 estimated a 30% probability of a negative leap second by 2035. A negative leap second, where a minute has 59 seconds, has never been applied. The conference calls it "a high risk of causing anomalies and disruption to critical infrastructures that are largely unprepared".

It also puts a price on preparing for one: an investment for time-dependent industries "similar to their preparations for the millennium bug". ITU-T, the International Union of Radio Science, the IEEE 1588 working groups for precision time and individual companies had all written to the BIPM asking for an earlier switch. The BIPM warns that some users might otherwise abandon UTC for satellite navigation time.

The inconsistency it removes

The resolution names a problem many operations teams know first-hand. Large service providers each handle a leap second in their own way, often by smearing the extra second across hours to avoid a step in their clocks, so systems synced to different providers disagree during the event. After May 2027 there will be no further events to smear. According to heise, there have been 27 leap seconds, the last at the end of 2016, and a one-hour adjustment would not be needed for more than a thousand years.

Radio time signals get longer. Under ITU-R Resolution 655, broadcasters whose equipment cannot carry the larger UT1-UTC difference may keep transmitting the old format until 2035, and no later than 2040.

Systems worth reviewing before May 2027

Most business software never sees a leap second; it reads a clock that the operating system and NTP keep correct. The places to look are those that handle the second explicitly: NTP and PTP servers configured to smear, applications that load the leap-second table, trading, telecoms and industrial systems that timestamp to the microsecond, and any code that converts between UTC and astronomical time. A configuration that assumes a future leap second will need changing, while one that simply follows its time source should need nothing.

The resolution asks the BIPM to work with the ITU, the IAU, the IUGG and the IERS to inform users, and says the IERS will change how it announces leap seconds. The French text is the official version.

Sources

• BIPM: CGPM Resolution 5 (2026), On the technical actions needed to ensure the continuity of UTC, 15 October 2026
• heise online: Nie mehr Schaltsekunden (in German), 15 October 2026