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The 2026 General Conference on Weights and Measures decided that UTC will become continuous on 20 May 2027, ending future leap-second adjustments. It set a maximum allowed difference of 3,600 seconds between UTC and Earth-rotation time UT1; the BIPM will monitor that difference and coordinate implementation with other organizations.
The General Conference on Weights and Measures (CGPM) has decided that Coordinated Universal Time (UTC) will become continuous on 20 May 2027, ending the addition of leap seconds to keep the official time scale aligned with Earth’s rotation. Under Resolution 5 of the conference’s 28th meeting, the permitted difference between UTC and Earth-rotation time UT1 will be capped at 3,600 seconds, or one hour.
The resolution says UTC will continue at the same rate as International Atomic Time (TAI), while no longer being adjusted by integral seconds to track UT1. The change alters the long-standing arrangement through which leap seconds have periodically kept civil time close to the time scale based on the angle of Earth’s rotation. UTC remains the internationally recommended time scale and the basis of civil time in most countries.
The CGPM said the one-hour ceiling on the UT1–UTC difference is intended to preserve UTC’s continuity for several centuries. It also directed the International Bureau of Weights and Measures (BIPM) to keep monitoring the difference and its projected evolution, report regularly to the International Committee for Weights and Measures, and work with organizations affected by the decision to inform users and support implementation.
The resolution cites the needs of industrial systems and critical infrastructure, including transportation, energy distribution, telecommunications and space projects, for a time scale without future adjustments. It also notes that organizations relying on UT1 can obtain information about its difference from UTC through Global Navigation Satellite Systems (GNSS) navigation messages and services provided by the International Earth Rotation and Reference Systems Service (IERS), national metrology institutes and others.
Continuous Time for Networked Systems
Stopping leap-second insertions is intended to remove a periodic adjustment that can require computer systems and services to handle a change in the clock. The resolution warns that the spread of Internet of Things devices, smart grids, distributed sensor networks and virtual services increases the risk of disruptions when a leap second is inserted, particularly if it is negative. It says global service providers have used different methods to manage leap seconds, which can produce inconsistent timing between systems and in disseminated time and frequency.
The change matters to infrastructure that uses precise timestamps to coordinate operations across networks. A continuous UTC could make that reference easier to handle, while the growing separation from UT1 means users needing Earth-rotation time will have to consult or calculate from the published difference. The decision does not make UTC and solar time identical: it allows their difference to grow, within the newly set limit.
The conference’s stated concern includes a possible negative leap second, which has never been applied. Citing estimates from a March 2025 workshop organized by the Consultative Committee for Time and Frequency and IERS, the resolution says the probability of one could reach 30% by 2035. It describes the risk of anomalies in infrastructure as high and says preparation would require financial investment, estimated to be similar in scale to preparations for the millennium bug. These are assessments recorded in the resolution, not a report that a negative leap second is scheduled.
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How the Leap-Second Policy Changed
UTC is produced by the BIPM and has the same rate as TAI, but historically has differed from it by an integral number of seconds. The leap-second system maintained an agreement between UTC and UT1, the time scale linked to Earth’s rotation. Because Earth’s rotation does not match atomic time exactly, leap seconds have been used to keep the difference within an agreed range.
At its 27th meeting in 2022, the CGPM decided that the maximum permitted 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. Resolution 5 of the 2026 meeting sets the implementation date earlier: 20 May 2027, and raises the maximum difference to one hour. The resolution also records that international telecommunications bodies had already been studying related changes to the transmission of UT1–UTC information, with a transition period contemplated for some radio equipment that cannot be replaced sooner.
The 2026 decision follows requests from industry and international organizations for a continuous reference time. The resolution names the International Telecommunication Union, the International Astronomical Union, the International Union of Geodesy and Geophysics, IERS and other groups as partners in the work ahead. It does not replace local time-zone rules or daylight-saving arrangements, which are separate civil-time decisions.
““continuous UTC will become effective on 20 May 2027””
— The 28th General Conference on Weights and Measures, Resolution 5
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Implementation and UT1 Reporting Details
The resolution sets the policy and effective date, but technical arrangements for every system are still being coordinated. It asks the BIPM to work with the ITU, IAU, IUGG and IERS and other affected organizations. The text also notes ongoing ITU-R work on how radio stations transmit the difference between UT1 and UTC, and describes a transition period for equipment that cannot be replaced earlier.
The resolution does not give a detailed schedule for individual industries, national services or equipment changes, nor does it specify how every user should obtain or apply UT1–UTC data. It calls for the BIPM to monitor the difference and its modeled evolution, but the resolution itself does not provide a projection of when the one-hour ceiling might be approached. The 30% figure refers to the estimated probability of a negative leap second by 2035 cited from the 2025 workshop; it does not mean that such an adjustment will occur.
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Coordination Before the 2027 Start
Before 20 May 2027, the BIPM is tasked with working with international standards and Earth-rotation organizations to inform users and support the decision’s application. IERS is expected, under the resolution’s stated support for its role, to take measures affecting its operational provision and announcement of leap seconds in line with the CGPM resolutions.
The BIPM will continue tracking UT1–UTC and report its evolution to the CIPM. Meanwhile, the ITU-R’s work on transmission of UT1–UTC information is relevant to users of radio systems, including cases where existing equipment may need more time to be replaced. The resolution establishes the policy date; further guidance and technical arrangements from the organizations involved will clarify how particular services and systems adapt.
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Key Questions
When will UTC stop using leap seconds?
The CGPM resolution says continuous UTC becomes effective on 20 May 2027. From that date, UTC will no longer be adjusted by leap seconds under the decision.
Does this mean UTC will stop being the international reference time?
No. The resolution continues to describe UTC as the internationally recommended time scale and the basis of civil time in most countries. The change concerns its adjustments to track Earth’s rotation.
What happens to the difference between UTC and Earth’s rotation?
The difference between UTC and UT1, a time scale linked to Earth’s rotation, will be allowed to grow, up to a maximum of 3,600 seconds, or one hour. Users who need UT1 can use published UT1–UTC information.
Was a negative leap second scheduled?
No. The resolution cites a workshop estimate that the probability of a negative leap second could reach 30% by 2035, but that is a risk estimate, not a scheduled event. The CGPM decision is intended to avoid future leap-second adjustments.
Source: hn
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