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A leap second is an occasional one-second adjustment to Coordinated Universal Time (UTC). It keeps the atomic time scale used by clocks and networks close to UT1, the time scale based on Earth’s actual rotation. Because Earth does not rotate at a perfectly constant rate, atomic time and rotation-based time slowly drift apart; the International Earth Rotation and Reference Systems Service (IERS) decides when a correction is needed.
In a positive leap second, the final minute of a UTC day contains 23:59:60 before the clock reaches 00:00:00 on the next date. The current framework keeps the absolute difference between UT1 and UTC below 0.9 seconds. IERS Bulletin C 72, dated 6 July 2026, says that no leap second will be inserted at the end of December 2026.
UTC and UT1 measure different kinds of time
UTC is the stable, internationally coordinated time scale used for civil timekeeping, computer systems, telecommunications and most clocks. Its seconds are based on atomic standards, so the duration of one second is highly uniform.
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UT1 is an astronomical time scale derived from observations of Earth’s rotation. It describes how the rotating Earth is oriented relative to the sky, which makes it useful for astronomy, Earth science and applications that must relate clock time to the apparent position of the Sun and stars.
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| Characteristic | UTC | UT1 |
|---|---|---|
| Basis | Atomic time standards | Measured Earth rotation |
| Main purpose | Stable, shared civil and technical timekeeping | Tracking Earth orientation and the relationship between time and the sky |
| Behavior | Seconds remain consistent | Intervals vary because Earth’s rotation is irregular |
| How they are linked | Occasional leap-second steps keep UTC close to UT1 | Provides the rotation reference against which UTC is checked |
The distinction matters because neither scale can simply replace the other. Atomic clocks provide the stability needed by modern systems, while UT1 preserves a connection between official time and the rotating Earth.
Why UTC and Earth-rotation time drift apart
Earth’s rotation changes from day to day. Tidal effects, movements in the atmosphere and oceans, exchanges of momentum within the Earth system and other geophysical processes can make a day slightly longer or shorter. There is a long-term tendency for rotation to slow, but the change is not a smooth, predictable rate that can be used as a fixed calendar rule.
The atomic second also has a historical origin: its duration was calibrated against Ephemeris Time rather than directly against mean solar time. As atomic clocks continued ticking uniformly while Earth’s rotation varied, the accumulated difference between UTC and UT1 changed in both size and direction.
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What a leap second looks like on a clock
Positive leap second
A positive leap second adds one labeled second at the end of a UTC day. The sequence is:
23:59:5923:59:6000:00:00on the next date
The label 23:59:60 is intentional. It identifies a real UTC second that does not exist on an ordinary day and allows the time scale to absorb one extra second without renumbering earlier timestamps.
Negative leap second
The rules also permit a negative leap second if Earth’s rotation makes UTC fall too far behind UT1. In that case, a second would be omitted: the clock would move from 23:59:58 directly to 00:00:00. No negative leap second has occurred so far.
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A leap second is not a leap year. Leap years follow a calendar rule that adds a day to keep seasons aligned with the calendar. Leap-second decisions depend on measured Earth rotation and can occur in different years, at irregular intervals.
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Who decides whether one is needed
The IERS measures Earth orientation and monitors UT1−UTC. Its operational objective is to keep the absolute difference below 0.9 seconds. When observations and forecasts show that a correction is required, the IERS announces it in Bulletin C.
Bulletin C is issued every six months. Each issue either announces a UTC step at the next eligible date or states that no step will occur then. Since the current system began, insertions have been scheduled at the end of June or the end of December, giving operators a defined boundary at which to prepare.
NIST explains that additions have usually been made when UTC is ahead of UT1 by about 0.4 seconds or more. That figure describes usual operational practice, not a guaranteed prediction formula. The formal requirement is the ±0.9-second tolerance, and the IERS decision depends on Earth-rotation measurements and forecasts.
The current schedule notice
IERS Bulletin C 72, issued on 6 July 2026 and signed by the director of the IERS Earth Orientation Center, states: NO leap second will be introduced at the end of December 2026.
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The same bulletin reports UTC−TAI = −37 seconds from 1 January 2017 until further notice. TAI is International Atomic Time, the continuous atomic reference from which UTC is offset by the accumulated leap-second history. The December 2026 statement applies to that announced date; a later Bulletin C can change the schedule for a subsequent June or December.
History of the system
Introduction in 1972
The present leap-second system began in 1972. The initial correction was 10 seconds, and the first inserted leap second occurred on 30 June 1972. Starting with that offset allowed UTC to move from earlier coordination arrangements into a system that could use atomic seconds while remaining close to UT1.
How often leap seconds occur
NIST reports that, since corrections began, leap seconds have occurred about once every 1.5 years on average. This is a historical average, not a timetable. Earth’s rotation can remain close enough to UTC for several years, or require a correction after a shorter interval. The average therefore cannot predict the next event.
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Removing leap seconds would make UTC a continuously ticking atomic scale, which would simplify some digital systems. Over a long period, however, UTC would drift farther from UT1 and from the day-night cycle. Astronomers and other users that currently rely on UTC as a convenient civil approximation to Earth-rotation time would need separate adjustments or conversion procedures.
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Keeping the scales close has practical value for observations and Earth-orientation work. The cost is that a normally uniform UTC timeline occasionally contains an exceptional label or a one-second step. The choice is therefore not between a perfect and an imperfect system; it is a trade-off between a stable continuous count and a civil time scale that remains tied to the rotating Earth.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What leap seconds mean for technology
Time stamps and data logs
Most software assumes that clock labels advance through ordinary minutes. A system that receives 23:59:60 must either represent that value explicitly, map it according to a documented policy or risk rejecting it, duplicating a timestamp or recording events out of order. Log formats, database constraints and monitoring tools should therefore specify how an exceptional UTC label is handled.
Telecommunications and time distribution
Networks distribute time to many devices, and a one-second step can arrive at slightly different moments at different points in the system. Unsynchronized handling can produce inconsistent ordering, temporary alarms or apparent clock reversals. Operators of time-distribution services need an announced event, an implementation plan and checks that dependent equipment follows the same convention.
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Operational preparation
Organizations that cannot tolerate an ambiguous second should identify every component that consumes UTC, verify whether it accepts 23:59:60, preserve an unambiguous event sequence and test recovery after the date boundary. The IERS announcement supplies the date; it does not prescribe one universal software method, so each system owner must document its own behavior.
The policy debate over future UTC
The irregular timing of leap seconds has led to proposals for changing the tolerance between UTC and UT1. A 2024 paper by Judah Levine and Patrizia Tavella, hosted by NIST, discusses allowing a larger maximum difference while retaining a link between the scales, potentially through an algorithmic adjustment. The authors emphasize that leap events are difficult to predict and that no single approach satisfies every community’s requirements.
An ITU account from 2015 describes the tension between a continuous reference for electronic navigation and computerized systems and the existing UTC arrangement. That account records a decision to continue studies; it is not an operational schedule. The currently announced schedule is the one published by the IERS in its Bulletins C.
How to interpret a leap-second announcement
- Check the issuing authority. Use the latest IERS Bulletin C rather than an old calendar or a recurring reminder.
- Check the date and direction. The bulletin will say whether a positive or negative step is planned and whether it occurs at the end of June or December.
- Check your system’s time scale. A device may display local civil time, UTC, TAI or another monotonic counter; the leap-second rule applies specifically to UTC.
- Check timestamp handling. Confirm how your software stores, compares and exports the exceptional second, and how it orders events around midnight.
- Record the bulletin version. Future notices can supersede a previous “no leap second” statement for a later eligible date.
Common points of confusion
| Question | Correct interpretation |
|---|---|
| Does a leap second happen every year? | No. It is irregular; the 1.5-year figure is only a historical average. |
| Is it decided by the calendar? | No. The IERS bases the decision on measured and predicted Earth rotation. |
| Does every day contain 86,401 seconds? | No. Only a day with a positive leap second has the extra labeled second. |
| Has a second ever been removed? | The rules allow it, but no negative leap second has occurred. |
| Is UTC the same as UT1? | No. UTC is atomic and stable; UT1 follows irregular Earth rotation. Leap seconds keep them within the permitted tolerance. |
A leap second is therefore a small, deliberate correction with a large systems implication: it preserves the usefulness of UTC as both a precise atomic scale and a practical approximation of Earth-rotation time. The IERS measurement and bulletin process determines when that compromise is needed, rather than any fixed annual schedule.
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