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The 28th General Conference on Weights and Measures has adopted a resolution making UTC continuous from May 20, 2027, ending future leap-second adjustments. It sets a maximum permitted difference between Earth-rotation time UT1 and UTC of 3,600 seconds, or one hour; implementation details for affected services and equipment remain under coordination.
The General Conference on Weights and Measures has decided that Coordinated Universal Time (UTC) will become continuous on May 20, 2027, ending the practice of adding leap seconds to keep civil time aligned with Earth’s rotation. Resolution 5 also sets a maximum difference of 3,600 seconds between UTC and UT1, the time scale that tracks Earth’s rotation, giving technical systems a fixed limit while removing future leap-second adjustments.
The decision was adopted as Resolution 5 at the 28th meeting of the CGPM in 2026. The conference said continuous UTC is needed because industrial users and operators of critical infrastructure—including transport, energy distribution, telecommunications and space projects—need a time reference that does not undergo sudden second-level adjustments. The resolution makes the change effective on a specific date rather than leaving it as a future policy goal.
UTC is produced by the International Bureau of Weights and Measures (BIPM) and is the international reference time scale and basis for civil time in most countries. It runs at the same rate as International Atomic Time, or TAI, while the historical leap-second system has kept it close to UT1, which reflects Earth’s changing rotation. Under the new arrangement, UTC will no longer be adjusted by leap seconds to maintain that close match. The permitted UT1–UTC difference will be capped at one hour.
The resolution directs the BIPM to work with organizations including the ITU, IAU, IUGG and IERS to inform users and support implementation. It also asks the BIPM to monitor the difference between UT1 and UTC, its projected evolution and modelling, and to report regularly to the International Committee for Weights and Measures. The supplied resolution text cuts off during a further request concerning regional metrology organizations, so its full wording is not available here.
Why Infrastructure Needs Continuous Time
Leap seconds can require computer clocks and connected systems to handle a time adjustment outside the steady progression many applications assume. The CGPM’s resolution says that this can create operational risks, particularly as systems become more connected through the Internet of Things, smart grids, distributed sensors and online services. A single shared standard matters because timing errors or inconsistent time references can affect how systems coordinate events and records.
The concern is not only the familiar addition of a second. The resolution cites expert estimates that the probability of a negative leap second—removing a second—could reach 30% by 2035. No negative leap second has been applied before, and the CGPM says critical infrastructure is largely unprepared for one. It also notes that service providers use different methods to manage leap seconds, potentially creating inconsistencies between systems and in the time being distributed.
The change does not make Earth’s rotation uniform or remove the need to know its relationship to time standards. Instead, it separates the continuous civil reference UTC from the variable rotation scale UT1, with a defined maximum offset. Users that need Earth-rotation information can obtain UT1–UTC data through satellite navigation messages and services provided by the IERS, national metrology institutes and others. The practical effect is a stable UTC feed, while the changing offset remains available to applications that require it.
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How the Leap-Second Policy Changed
The CGPM had already set the direction of the change. In 2018, Resolution 2 confirmed UTC’s relationship to TAI and its role as the recommended international reference. In 2022, Resolution 4 recognized that UTC’s integral-second offset from TAI was maintained to keep it in agreement with Earth-rotation time, and called for the maximum UT1–UTC difference to be increased in or before 2035 so UTC could remain continuous for at least a century.
The 2026 resolution turns that earlier decision into a schedule and a numerical limit. It sets the effective date at May 20, 2027 and allows the offset to reach one hour, which the conference says supports UTC continuity for several centuries. The prior 2022 decision had left the timing of the increase to be settled later.
Radio distribution also forms part of the transition. The resolution cites ITU-R Resolution 655, which provides for a transition period to allow dissemination of the increased UT1–UTC difference through radio systems until 2035, or no later than 2040 where existing equipment cannot be replaced sooner. The ITU-R has been studying possible updates to radio transmission codes. The CGPM also records support for continuous UTC from international organizations and industry representatives.
““continuous UTC will become effective on 20 May 2027””
— The 28th General Conference on Weights and Measures, Resolution 5
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Remaining Work Before the Change
The decision sets the policy, effective date and maximum offset, but it does not by itself specify every technical change required across systems that distribute or consume UTC. Radio transmission updates are still being studied by ITU-R, and the resolution recognizes that some older equipment may need a longer replacement period. The exact implementation schedules for individual countries, providers and infrastructure operators are not detailed in the supplied resolution.
It is also not clear from the resolution how quickly different services will adopt the updated dissemination methods, or what transitional practices each will use. The one-hour cap is a maximum, not a prediction that UT1 and UTC will immediately differ by an hour. The actual offset will continue to depend on Earth’s rotation and will be monitored by the IERS and BIPM. No negative leap second has been applied to date; the cited 30% figure is an expert estimate for the probability by 2035.
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Coordination Toward May 2027
The BIPM is tasked with coordinating with the ITU, IAU, IUGG, IERS and other affected organizations to inform users and support the change. It will continue tracking UT1–UTC and its estimated evolution, then report that information regularly to the CIPM. The IERS is expected to take the measures needed to align its operational provision and announcements of leap seconds with the CGPM resolutions.
Radio-system updates and equipment replacement will proceed under the transition arrangements referenced in the resolution, including the allowance for some systems to continue using older equipment until no later than 2040 where earlier replacement is not possible. The stated milestone is May 20, 2027, when continuous UTC takes effect. The resolution does not provide a detailed implementation checklist for every user, so operators will need further guidance from the relevant standards and service organizations.
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Key Questions
What did the BIPM and CGPM decide?
The 28th CGPM adopted Resolution 5, setting continuous UTC to take effect on May 20, 2027. Under the policy, UTC will no longer receive future leap-second adjustments.
Will clocks stop tracking Earth’s rotation?
No. The relationship remains measurable through UT1–UTC, but UTC will not be adjusted to keep that difference close to zero. The resolution sets a maximum permitted difference of one hour, and UT1 information will remain available through services and navigation messages.
Why is the change being made?
The CGPM says leap-second adjustments can create risks for computer systems and critical infrastructure, especially when systems handle the adjustment differently. It also cites concern about a possible negative leap second, which has never been applied and for which the resolution says many systems are unprepared.
When does continuous UTC begin?
The effective date specified in Resolution 5 is May 20, 2027. The resolution also describes a transition period for some radio systems, with certain existing equipment potentially remaining in use until no later than 2040.
Does the decision mean UT1 and UTC will immediately differ by an hour?
No. One hour is the maximum allowed difference, not the expected offset on the start date. The actual difference will vary with Earth’s rotation and will continue to be monitored.
Source: hn
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