BIPM Decides To Stop Adding Leap Seconds To UTC

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The General Conference on Weights and Measures has adopted a resolution making UTC continuous from May 20, 2027, ending the use of leap seconds to keep it aligned with Earth’s rotation. The resolution permits the difference between UTC and UT1 to grow to as much as one hour; it does not change civil clocks by an hour on that date.

The General Conference on Weights and Measures (CGPM) has decided that Coordinated Universal Time (UTC) will become continuous on May 20, 2027, ending the addition of leap seconds used to keep the clock scale close to Earth’s rotation. Its 2026 Resolution 5 sets a new maximum of 3,600 seconds, or one hour, for the difference between UTC and UT1, the time scale tied to Earth’s rotation.

The resolution was adopted at the 28th meeting of the CGPM, the international body responsible for decisions on the SI measurement system and related standards. It directs the BIPM, which produces UTC, to implement continuous UTC on the specified date. UTC will retain the same rate as International Atomic Time (TAI); the change concerns the adjustments that have historically kept UTC aligned with UT1.

Under the existing approach, leap seconds are inserted to prevent the gap between UTC and UT1 from exceeding a defined limit. The new limit is one hour, a major increase from the previous constraint. The resolution says this is intended to support UTC’s long-term continuity for several centuries. It does not prescribe adding or subtracting an hour from clocks in 2027: the number describes the maximum permitted divergence between two time scales, not a scheduled change to local civil time.

The BIPM is asked to work with organizations including the ITU, IAU, IUGG and IERS to inform users and support implementation. It must also continue monitoring the difference between UT1 and UTC and report its evolution to the International Committee for Weights and Measures (CIPM). The resolution reflects the needs of systems that require a continuous time reference, including transport, energy distribution, telecommunications and space operations.

Why Continuous UTC Matters to Systems

Many digital and industrial systems depend on timestamps to order events, synchronize equipment and coordinate services. A leap second is a small adjustment in clock time, but how software handles it can differ. The CGPM resolution says varying mitigation methods used by service providers can create inconsistencies between systems and in disseminated time and frequency. Removing future leap-second insertions is meant to give those users a continuous reference rather than a clock scale that periodically steps.

The decision also addresses the possibility of a negative leap second, which would remove a second rather than add one. The resolution cites a March 2025 workshop organized by the Consultative Committee for Time and Frequency and IERS, where experts estimated the probability of such an adjustment could reach 30% by 2035. No negative leap second has previously been applied. The conference said critical infrastructure is largely unprepared for one and that preparing for it could require substantial investment.

For most people, the immediate effect is not a visible change to a watch or phone. The broader consequence is a change in the relationship between the internationally coordinated civil time standard and Earth’s variable rotation. Users who need Earth-rotation information will still be able to obtain UT1–UTC values through sources such as satellite navigation messages and services provided by IERS and national metrology institutes.

From Leap Seconds to a Wider Limit

UTC is produced by the BIPM and has the same rate as TAI, but differs from it by an integral number of seconds. UTC is the recommended international reference time scale and underpins civil time in most countries. The integral-second offset has been maintained to keep UTC near UT1, which describes Earth’s rotation and is not perfectly uniform.

At its 26th meeting in 2018, the CGPM reaffirmed the definition of UTC. At the 27th meeting in 2022, it decided that the maximum allowed difference between UT1 and UTC would be increased in or before 2035, with the aim of ensuring UTC’s continuity for at least a century, and asked for a further resolution. Resolution 5 now brings the change forward: continuous UTC takes effect in 2027, with a one-hour maximum difference.

The resolution also points to preparations in radio communications. ITU-R has been studying possible updates to how radio systems transmit the UT1–UTC difference. A 2023 World Radiocommunication Conference resolution provided for a transition period, allowing the larger difference to be disseminated through radiocommunication systems until 2035, and no later than 2040, where existing equipment cannot be replaced sooner.

““continuous UTC will become effective on 20 May 2027””

— The 28th General Conference on Weights and Measures, Resolution 5

Implementation and Earth-Rotation Tracking

The resolution sets the effective date and maximum UT1–UTC difference, but the source text does not provide a detailed implementation schedule for every affected system or a list of equipment changes required in each sector. The transition for radio systems may vary where older equipment cannot be replaced promptly, under the timetable described in the resolution.

It is also not possible to state how quickly the actual UT1–UTC difference will grow or when it might approach the new one-hour limit. That depends on Earth’s rotation and must be monitored; the resolution assigns the BIPM and partner bodies continuing monitoring and communication duties. The 30% figure cited for a possible negative leap second by 2035 is an estimate reported from the 2025 expert workshop, not a certainty or a forecast that such a second will occur.

Preparation Before the 2027 Change

Before May 20, 2027, the BIPM is expected to coordinate with international organizations and metrology bodies to explain the change and support users. IERS will adjust its operational provision and announcements about leap seconds in line with the CGPM decisions, while relevant telecommunications groups continue work on disseminating UT1–UTC information through radio systems.

The BIPM will keep tracking the difference between UT1 and UTC and report its evolution to the CIPM. Operators of time-dependent systems will need to assess how their equipment and software use UTC and, where required, obtain UT1 information separately. The resolution does not specify individual organizations’ implementation plans, so the pace and details of preparation remain to be established.

Key Questions

Will clocks be moved by an hour on May 20, 2027?

No. The one-hour figure is the new maximum permitted difference between UTC and UT1. The resolution does not call for moving civil clocks forward or backward by an hour on its effective date.

What is a leap second?

A leap second is an adjustment to UTC used to keep it close to UT1, a time scale based on Earth’s rotation. The CGPM decision makes UTC continuous from May 20, 2027, so future leap seconds will no longer be added to UTC under this arrangement.

What is the difference between UTC and UT1?

UTC is the international time scale produced by the BIPM and used as the basis for civil time in most countries. UT1 describes Earth’s rotation. The resolution allows their difference to reach as much as one hour and says that value will continue to be monitored.

Why did the CGPM decide to stop leap seconds?

The resolution cites the need for uninterrupted time in systems such as transport, energy distribution, telecommunications and space operations. It also notes concerns that different ways of handling leap seconds can produce timing inconsistencies, and that critical infrastructure may be unprepared for a negative leap second.

How will people find out how far UT1 differs from UTC?

The resolution points to GNSS navigation messages and services from IERS, national metrology institutes and other providers as sources of UT1–UTC information. It also refers to ongoing international work on transmitting that difference through radio systems.

Source: hn