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The General Conference on Weights and Measures has decided that UTC will become a continuous time scale on May 20, 2027, ending the practice of adding leap seconds to keep it closely aligned with Earth’s rotation. The permitted difference between UTC and UT1, the time scale tied to Earth’s rotation, will rise to one hour; the BIPM will work with international organizations on implementation and user information.
The General Conference on Weights and Measures (CGPM) has decided that Coordinated Universal Time (UTC) will become continuous on May 20, 2027, ending future leap-second adjustments. Under Resolution 5, the permitted gap between UTC and UT1, which tracks Earth’s rotation, will increase to 3,600 seconds, or one hour—a change intended to give computer systems and other users a stable international time reference.
The resolution was adopted at the 28th CGPM meeting in 2026. It sets a specific start date for continuous UTC, following a 2022 decision that called for increasing the allowed difference between UTC and UT1 in or before 2035. The new resolution brings that change forward: continuous UTC takes effect May 20, 2027.
UTC is produced by the International Bureau of Weights and Measures (BIPM) at the same rate as International Atomic Time (TAI). Historically, leap seconds have been used to keep UTC within a set offset of UT1, which reflects the Earth’s rotation. After the change, UTC will not receive leap seconds; the resolution instead sets a maximum |UT1−UTC| difference of one hour.
The decision directs the BIPM to keep monitoring the difference and its projected evolution, and to report regularly to the International Committee for Weights and Measures (CIPM). It also calls for continued work with the ITU, IAU, IUGG and IERS, among other organizations, to inform users and support implementation. The supplied resolution text ends during its list of instructions, so further details from that list are not available here.
A Stable Clock for Digital Systems
UTC is the international reference time scale and forms the basis of civil time in most countries. Its continuity matters to services that coordinate events across networks, infrastructure and borders, including telecommunications, energy distribution, transport and space projects. A leap second can require systems to handle an exceptional change to their clocks, and the resolution says different mitigation methods used by service providers can leave timing inconsistent across systems.
The change does not alter civil time zones or stop Earth’s rotation from affecting the relationship between clocks and the apparent solar day. Rather, it removes leap-second adjustments from UTC and permits the separation between UTC and Earth-rotation time UT1 to grow, up to the stated limit. Users who need UT1 information will still be able to obtain the difference through sources such as satellite navigation messages and services provided by the IERS and national metrology institutes, according to the resolution.
For infrastructure operators, the decision offers a defined standard rather than a need to prepare for another leap-second insertion. Its practical importance depends on systems adopting the change consistently and on the continued availability of information for users who need Earth-rotation time.
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From Leap Seconds to a Set Limit
The CGPM’s 2018 resolution described UTC as a time scale produced by the BIPM, running at the same rate as TAI and differing from it by an integral number of seconds. The offset between UTC and Earth-rotation time has been managed through leap seconds. The 2022 CGPM resolution recognized that this arrangement would need to change and called for a larger maximum difference, with a target date of 2035 or earlier.
Resolution 5 cites a March 2025 workshop organized by the Consultative Committee for Time and Frequency and the IERS. The workshop estimated that the probability of needing a negative leap second could reach 30% by 2035. A negative leap second has never been applied, according to the resolution, and experts considered it a risk for critical systems that were largely unprepared.
The document also points to the difficulty of coordinating leap-second handling across global services and the growing use of connected devices, distributed sensors and smart grids. Separately, the International Telecommunication Union’s radiocommunication sector has been studying how radio systems transmit the UT1−UTC difference. A related resolution allows for a transition period for some existing equipment, potentially extending until 2040 in specified cases.
““continuous UTC will become effective on 20 May 2027””
— The 28th General Conference on Weights and Measures, Resolution 5
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How Users Will Adapt Systems
The resolution sets the effective date and the one-hour limit, but the supplied text does not provide a detailed technical schedule for every affected system or explain how individual operators should update equipment and software. The BIPM is tasked with working with international bodies to inform users, while the ITU-R has been studying possible updates to radio transmission codes for the UT1−UTC difference.
It is also not clear from the supplied text how every service will present or distribute UT1 information after the change, or whether all affected organizations will be ready by May 2027. The cited 30% estimate for a negative leap second by 2035 is a probability assessment from a 2025 workshop, not a prediction that such an adjustment would certainly have been required. The resolution’s stated one-hour cap also means UTC and UT1 will remain related, but no longer kept close through leap seconds.
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Implementation Before May 2027
Before May 20, 2027, the BIPM says it will continue coordinating with the ITU, the International Astronomical Union, the International Union of Geodesy and Geophysics, the IERS and other affected organizations. Its assigned work includes monitoring the value and projected evolution of UT1−UTC and regularly informing the CIPM.
Users of time services and operators of systems that depend on UTC will need to follow guidance from relevant standards bodies and providers as implementation details are developed. The resolution notes that some radio equipment may require a transition period to transmit the increased UT1−UTC difference; the cited international framework allows such a period until 2035, and in certain cases no later than 2040, where existing equipment cannot be replaced sooner. The immediate milestone is the resolution’s May 2027 effective date; further operational guidance is expected through the organizations named in the decision.
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Key Questions
When will UTC stop receiving leap seconds?
The resolution says continuous UTC takes effect on May 20, 2027. It establishes that date as the start of the new arrangement.
Will clocks stop matching Earth’s rotation?
No. UTC will no longer be adjusted with leap seconds, but the difference between UTC and UT1, the time scale tied to Earth’s rotation, will be monitored and capped at one hour under the resolution. Users needing UT1 information can use specified services and navigation messages.
Why did the CGPM decide to end leap-second adjustments?
The resolution cites risks of disruption to infrastructure and inconsistent timing caused by different ways of handling leap seconds. It also points to the possibility of a negative leap second, which has not previously been applied and was considered difficult for unprepared systems.
Will the change affect local time zones?
The resolution changes the relationship between UTC and UT1; it does not announce changes to time zones or daylight-saving rules. Civil-time policies remain decisions made by the relevant jurisdictions.
Source: hn
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