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A leap second is an occasional one-second adjustment to Coordinated Universal Time (UTC). It keeps UTC close to UT1, the astronomical time scale based on Earth’s actual rotation. Atomic clocks produce extremely stable seconds, but Earth does not rotate at a perfectly steady rate, so the two time scales gradually drift apart. The International Earth Rotation and Reference Systems Service (IERS) measures that difference and decides when UTC needs a correction.
The current system does not follow a fixed calendar. IERS Bulletin C 72, dated 6 July 2026, says that no leap second will be introduced at the end of December 2026. The bulletin also reports UTC−TAI = −37 seconds from 1 January 2017 until further notice.
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What a leap second actually is
UTC is the world’s primary civil time standard. Its seconds are derived from atomic time, so they are uniform and predictable. That stability is essential for computers, telecommunications, navigation, scientific instruments and legal or financial timestamps.
UT1 serves a different purpose. It represents the orientation of Earth in space as inferred from observations of Earth’s rotation. Because the planet’s rotation varies, UT1 does not advance at exactly the same rate as atomic time. The difference between UT1 and UTC therefore changes over time.
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A leap second is the occasional step inserted into UTC to prevent that difference from becoming too large. Under the current framework, the absolute value of UT1−UTC is kept below 0.9 seconds. The adjustment preserves a practical connection between atomic civil time and the position of the Sun and stars in the sky.
NIST’s plain-language definition is: “A leap second is a second added to Coordinated Universal Time (UTC) in order to keep it synchronized with astronomical time.” That describes the purpose, but not a regular schedule: the timing depends on measured Earth rotation.
UTC and UT1 compared
| Time scale | What it is based on | Main purpose | How it behaves | Connection between the scales |
|---|---|---|---|---|
| UTC | Stable atomic seconds | General civil and technical timekeeping | Uniform except for occasional one-second steps | Adjusted when needed to stay close to UT1 |
| UT1 | Earth’s observed rotation | Earth orientation and the relationship between time and the apparent sky | Irregular because Earth’s rotation varies | Measured against UTC; the absolute difference is kept below 0.9 seconds |
Why atomic time and Earth-rotation time diverge
The modern second is defined so that atomic clocks can reproduce it consistently. Earth’s rotation, by contrast, is affected by physical processes inside and outside the planet. It has a long-term tendency to slow, but the rate is not constant: shorter-term variations can make the rotation speed up or slow down relative to the average.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsNIST identifies a historical reason as well as the physical one. The atomic second inherited a duration calibrated against Ephemeris Time rather than mean solar time. As Earth’s rotation changed, that inherited atomic interval and the solar day stopped matching perfectly. UTC consequently needs occasional corrections if it is to remain both technologically stable and astronomically meaningful.
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Without leap seconds, UTC would steadily move farther from UT1. The difference would not make atomic clocks inaccurate; it would mean that a UTC time near noon would gradually lose its close relationship with the Sun being near its highest point. Leap seconds are the mechanism that limits that separation under the present rules.
What a positive leap second looks like
A positive leap second inserts an extra label at the end of a UTC day. The clock sequence is:
- 23:59:59
- 23:59:60
- 00:00:00 on the next date
The label 23:59:60 is therefore valid in UTC on a positive-leap-second date. It is not a time that occurs every day, and it should not be confused with a time-zone change or a daylight-saving transition.
A negative leap second is allowed in principle if Earth’s rotation makes one necessary. It would remove a second from the UTC sequence, but no negative leap second has occurred so far. The possibility is part of the rules even though all leap seconds used to date have been additions.
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Leap seconds also differ from leap years. A leap year follows a calendar rule that adds a day on a predictable cycle. A leap second responds to measurements of Earth orientation, so its date and even its occurrence are irregular.
Who decides, and how often can it happen?
The IERS is responsible for monitoring Earth rotation and deciding whether UTC needs a step. Its Earth Orientation Center publishes Bulletin C every six months. Each bulletin either announces a leap second for the next eligible date or confirms that none will occur then.
Since the system began, leap seconds have been placed at the end of June or December. Those are permitted insertion points, not guaranteed events. A June or December bulletin can say that no adjustment is required, and many such dates pass without a leap second.
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The tolerance and the usual operational trigger
The formal operating objective is to keep UT1−UTC within ±0.9 seconds, or, stated as an absolute value, below 0.9 seconds. NIST explains that additions usually occur when UTC is ahead of UT1 by about 0.4 seconds or more. That 0.4-second figure describes usual practice; it is not a guaranteed prediction formula and does not replace the formal tolerance.
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Because Earth’s rotation is irregular, the interval between events cannot be calculated from a calendar. NIST reports a historical average of about one leap second every 1.5 years since corrections began in 1972. That is an average over the past, not a forecast of the next adjustment.
Historical milestones and the current notice
| Date or period | What happened |
|---|---|
| 1972 | The current leap-second system began. The initial correction was 10 seconds. |
| 30 June 1972 | The first leap second was inserted. |
| Since 1972 | NIST gives a historical average of roughly one leap second every 1.5 years; the spacing has been irregular. |
| 1 January 2017 onward | IERS Bulletin C 72 reports UTC−TAI = −37 seconds until further notice. |
| 6 July 2026 | IERS Bulletin C 72 announced that no leap second will be introduced at the end of December 2026. |
The 2026 statement is a scheduling notice, not a permanent guarantee. A later Bulletin C can announce a different decision for a future June or December date.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why leap seconds matter to technology
For astronomy and Earth science, keeping UTC near UT1 preserves a usable link between civil timestamps and Earth’s orientation. Observations, pointing calculations and records that relate events to the apparent sky benefit from that relationship.
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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →The same discontinuity can be awkward for engineered systems. A clock that normally advances through consecutive seconds must handle the exceptional label 23:59:60. Data logging, telecommunications and time-distribution services may have to decide how to represent, transmit, order or validate that extra second.
The risk is not that a leap second makes every computer clock wrong. The risk is that different components may have incompatible assumptions about a day containing 86,401 labeled seconds, or may reject a timestamp they were built to consider impossible. A system that silently collapses the extra second, duplicates an ordinary timestamp, or applies a local workaround can create ordering and reconciliation problems.
Practical checks for software and operations teams
- Identify whether each service stores UTC, a monotonic elapsed-time value, local civil time or an astronomical scale such as UT1. They are not interchangeable.
- Check timestamp parsers, validators, databases and message formats for their treatment of
23:59:60. - Keep the IERS Bulletin C schedule in operational planning rather than assuming that every June or December contains a leap second.
- Test log ordering, time synchronization and cross-system reconciliation around an announced event.
- Record the time scale and any conversion policy in technical documentation so that a later reader can interpret the timestamp correctly.
These checks matter most where many systems exchange time-sensitive records. A single application may appear to work while an upstream clock, database or network service handles the exceptional second differently.
Why some experts want a different system
Leap seconds preserve the UTC–UT1 relationship, but their irregular timing is difficult for systems designed around continuous, monotonically advancing time. This has produced a long-running policy debate rather than a settled replacement.
A 2024 paper by Judah Levine and Patrizia Tavella, hosted by NIST, discusses proposals to permit a larger maximum UTC–UT1 difference while retaining a link between the scales. One possibility is an algorithmic adjustment rather than an unpredictable one-second event. The paper presents this as an unresolved design problem: no single approach meets every community’s requirements.
The ITU’s 2015 account likewise describes tension between a continuous reference time favored by electronic navigation and computerized systems and the existing UTC approach. Its record concerns a decision to continue studies, not a new operating schedule. The current schedule remains the one communicated by IERS bulletins, including Bulletin C 72 for 2026.
How to interpret an IERS announcement
- Find the bulletin date. Bulletin C is issued every six months, so the date tells you how current the decision is.
- Read the explicit decision. The bulletin will say whether a leap second is planned at the next eligible June or December month-end.
- Check the stated UTC–TAI relationship. Bulletin C 72, for example, states UTC−TAI = −37 seconds from 1 January 2017 until further notice.
- Apply the result to the correct time scale. A UTC adjustment does not mean that every local clock or every elapsed-time counter should be altered in the same way.
For the latest notice covered here, the direct answer is “none at the end of December 2026.” That answer is valid for the date specified in Bulletin C 72 and should not be generalized to later eligible dates.
The essential distinction
UTC is designed to give the world a stable atomic time base while remaining close enough to Earth’s rotation for civil time to track the astronomical day. UT1 supplies the Earth-rotation measurement; IERS compares it with UTC and occasionally inserts a leap second when the permitted difference requires correction. The result is a compromise: predictable atomic seconds most of the time, with an irregular one-second adjustment when the planet’s rotation makes it necessary.
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