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Leap seconds are being phased out, but not in 2035. The General Conference on Weights and Measures (CGPM) has decided that a continuous form of Coordinated Universal Time (UTC) will take effect on 20 May 2027. From then on, UTC will no longer insert occasional one-second jumps to stay within a fraction of a second of Earth’s irregular rotation. The change is designed to make computer, satellite, telecommunications and power-grid timing more predictable, while requiring astronomers and other users of Earth-rotation time to work with published UT1 data.
What is a leap second?
A leap second is a one-second adjustment to UTC, the international reference time used for civil clocks and digital systems. Earth does not rotate at a perfectly constant rate, so astronomers track its actual rotation with a scale called UT1. Historically, international timekeepers inserted a leap second whenever the difference between UTC and UT1 approached 0.9 seconds, as explained by the National Institute of Standards and Technology (NIST).
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A positive leap second is added at the end of June or December. The clock sequence becomes 23:59:59, 23:59:60, 00:00:00. A negative leap second would instead omit a second, jumping from 23:59:58 to 00:00:00. No negative leap second has ever been used.
When will leap seconds stop?
Continuous UTC becomes effective on 20 May 2027. That is the operative date in CGPM Resolution 5 (2026), which says, “continuous UTC will become effective on 20 May 2027.”
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Confusion comes from an earlier decision. Resolution 4 of the 27th CGPM (2022) set a target of increasing the permitted UT1−UTC difference in or before 2035. That was a policy deadline for changing the tolerance, not the date on which leap seconds would necessarily end. The 2026 resolution now establishes the nearer, specific effective date.
What changes in UTC after May 2027?
The new system will allow the absolute difference between UT1 and UTC to grow as large as 3,600 seconds (one hour). The 2026 resolution says this limit is intended to support continuity for several centuries. UTC will remain a single, continuous timescale rather than being interrupted by leap-second insertions.
That does not make Earth’s rotation constant, and it does not eliminate UT1. The NIST leap-second and UT1-UTC page will continue to provide Earth-rotation information. Systems that need the angle of the Earth relative to the stars can use UT1 or an appropriate correction instead of assuming UTC is always within a second of it.
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Why are leap seconds being phased out?
The central issue is not that a one-second change is noticeable to people looking at a wall clock. It is that modern infrastructure must represent, distribute and apply the change consistently across millions of devices.
The BIPM’s 2022 resolution identifies risks to critical digital infrastructure, including global navigation satellite systems (GNSS), telecommunications and energy transmission. Operators have developed different ways to handle leap seconds, and the lack of one common implementation can create synchronization failures or ambiguous timestamps.
The 2026 resolution also highlights the specific danger of a negative leap second. Because one has never been applied, software and equipment may not handle a missing second safely. A March 2025 workshop organized by the Consultative Committee for Time and Frequency and the International Earth Rotation and Reference Systems Service estimated a 30% probability of a negative leap second by 2035. That is a workshop estimate recorded by the BIPM, not a prediction that one will definitely occur.
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What does the decision mean for computers and networks?
For most networked systems, a continuous UTC scale removes an awkward exception. Software will no longer need special logic for a timestamp such as 23:59:60, and distributed services will not have to choose among different leap-second smearing, stepping or holdover strategies. More predictable time can simplify logging, transaction ordering, authentication, data exchange and synchronization between geographically separated systems.
The transition is not cost-free. NIST notes that timestamps, legacy equipment and software synchronized to UTC for astronomical observations may need adjustments. Operators should identify where applications assume UTC and UT1 differ by less than a second, document whether their time source supplies UTC, UT1 or another realization, and check vendor guidance before the effective date.
What happens to astronomy and other Earth-rotation applications?
Astronomy, geodesy, satellite operations and some surveying tasks use Earth orientation, not merely civil time. After 2027, UTC will gradually drift farther from UT1, so a UTC timestamp alone will provide less information about the Earth’s instantaneous rotation.
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Those users are not losing access to the needed data. International services will continue publishing UT1 and related Earth-orientation parameters. Professional systems may need updated software, new data feeds or revised timestamp conversions so observations are tied to UT1 when that is the relevant scale.
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What is the trade-off between continuous UTC and leap seconds?
| Priority | Traditional leap-second UTC | Continuous UTC from 20 May 2027 |
|---|---|---|
| Digital continuity | Requires special handling for occasional discontinuities. | Provides a continuously advancing civil timescale with no leap-second jumps. |
| Closeness to Earth rotation | Keeps UTC within 0.9 seconds of UT1. | Permits |UT1−UTC| to reach 3,600 seconds over time. |
| Operational risk | Different infrastructure operators may implement an insertion differently; a negative leap second would be especially unfamiliar. | Removes leap-second events but requires migration of systems that infer Earth rotation from UTC. |
What is the current UT1−UTC correction?
NIST lists a broadcast DUT1 correction of −0.1 seconds effective 29 October 2026. That is a dated operational value for the current system, not the offset that will exist after continuous UTC begins. The eventual difference will depend on how Earth’s rotation evolves and will be tracked through published Earth-orientation data.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should an organization do before 2027?
- Inventory time dependencies: Find applications, databases, embedded devices and monitoring tools that parse UTC, DUT1, leap-second tables or the value 23:59:60.
- Separate civil time from Earth orientation: Decide whether each process needs continuous UTC, UT1, or another technical timescale.
- Check synchronization sources: Confirm how GNSS receivers, NTP/PTP services and vendor appliances will represent the post-2027 relationship between UTC and UT1.
- Review historical data: Preserve unambiguous scale and offset metadata when combining records created before and after the transition.
- Test with suppliers: Obtain migration notes and firmware or software requirements from the manufacturers of timing hardware and critical applications.
Organizations should avoid treating the 2026 date as a routine clock update. The relevant question is which systems need a stable civil timescale and which need an explicit Earth-rotation correction.
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Will people notice the change?
Most people will not see a visible difference in everyday clocks, phones or calendars. The change matters mainly to infrastructure that distributes precise time and to scientific or industrial systems that currently rely on UTC’s tight alignment with UT1. Over many years, the separation between civil UTC and solar or rotational time will become measurable, but ordinary civil time will remain consistent and continuous.
Frequently Asked Questions
Will UTC become the same as atomic time?
UTC is already generated from atomic clocks, with historical leap seconds used to keep it near UT1. After 2027 it will remain an atomic-clock-based civil scale, but it will be allowed to drift farther from Earth-rotation time.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsCan a leap second still be added after 20 May 2027?
The 2026 CGPM decision establishes continuous UTC and a much larger permitted UT1−UTC range, so the former subsecond leap-second procedure is no longer used. Earth-rotation information will instead be distributed through UT1 and related corrections.
Does this change affect GPS or satellite navigation immediately?
The decision is intended to reduce discontinuity risk for GNSS and other synchronized infrastructure, but individual systems still need updates and clear definitions of their time scales. Operators should follow the service or equipment provider’s transition documentation rather than assume every receiver changes identically.
Why is 2035 often mentioned in articles about leap seconds?
The 2022 CGPM resolution called for the permitted UT1−UTC difference to be increased in or before 2035. The later 2026 resolution supersedes that uncertainty with a specific effective date: 20 May 2027.
