The Leap Second Is Being Retired. UTC Goes Continuous on May 20, 2027

Anyone who has chased a strange outage back to a 61-second minute will like this one. On October 15, the General Conference on Weights and Measures (CGPM), the treaty body that oversees the International System of Units, adopted a resolution at its 28th meeting in Versailles that ends leap seconds in Coordinated Universal Time (UTC).

Resolution 5 decides that "continuous UTC will become effective on 20 May 2027" and that "the maximum value for the difference |UT1-UTC| will be 3 600 seconds (1 hour), ensuring the long term continuity for UTC for several centuries." In plain terms, UTC will stop being nudged to follow the Earth's slightly irregular spin. Atomic time and solar time will be allowed to drift apart by up to an hour, which at the current pace is not expected to matter for a very long time.

Why now: the negative leap second

Since 1972, UTC has been kept within 0.9 seconds of UT1, the time scale based on the Earth's rotation, by inserting extra seconds. The last one was added at the end of 2016, and the IERS Bulletin C issued in July confirms no leap second at the end of 2026, leaving UTC 37 seconds behind International Atomic Time.

The trouble is that the Earth has been spinning slightly faster lately. The resolution cites a March 2025 workshop of the Consultative Committee for Time and Frequency (CCTF) and the IERS that estimated "the probability of a negative leap second will increase rapidly in the near future, reaching a 30 % probability by 2035." A negative leap second, a minute with only 59 seconds, has never been applied. The resolution calls it "a high risk of causing anomalies and disruption to critical infrastructures that are largely unprepared" and says preparing for it would cost industry an amount "similar to their preparations for the millennium bug."

The CCTF's president, Noël Dimarcq, said in a statement after the vote that the committee proposed to accelerate the process because of that risk. The 2022 CGPM had only committed to a change "in, or before, 2035." The new date pulls it forward by up to eight years.

What it means for networks and servers

For network and systems people, leap seconds have been a recurring headache: NTP leap indicator flags, kernel bugs, and big providers each "smearing" the extra second in their own way. The resolution acknowledges that last point directly, saying that different mitigation methods used by global service providers "leads to inconsistency in timing between different systems." It also warns that further delay could push some organizations to switch their reference to a GNSS time scale instead of UTC.

A continuous UTC removes that whole class of event. Nothing changes in the 37-second offset already baked in; there will simply be no new steps after May 20, 2027. Radio time signals are the slower part. The resolution notes that ITU-R Working Party 7A is studying how to transmit a larger UT1-UTC difference, and that ITU Resolution 655 allows a transition period for older equipment "until 2035, but no later than 2040."

Practical notes

• Leap seconds can only be added at the end of June or December. Bulletin C has already ruled one out for December 2026, and the next possible date under the old rules, the end of June 2027, falls after the cutover.
• Code that handles leap seconds does not need to be ripped out, but tests that assume future leap seconds, or tables that expect new entries, should be reviewed over time.
• Anyone who actually needs Earth-rotation time, such as astronomy or some navigation uses, can keep getting UT1-UTC from GNSS navigation messages and IERS services, as the resolution points out.

After roughly three decades of argument, the leap second now has an end date. For most of the internet, the best outcome is that nobody notices.