Coordinated Universal Time (UTC) is the internationally coordinated reference time scale used as the basis for standard time signals and civil time. The Bureau International des Poids et Mesures (BIPM) computes it from clock and time-transfer data contributed by laboratories worldwide; it is not the reading of a single master clock.
What UTC means and how it is made
UTC provides a common reference for coordinating time across borders and systems. The BIPM calculates it from international atomic-clock and time-transfer data. On its current UTC page, accessed in 2026, the BIPM describes contributions from around 80 laboratories and approximately 450 atomic clocks. Those figures are approximate, not a fixed or independently verified count. BIPM: Time metrology
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Laboratories also maintain local, real-time versions called UTC(k), where “k” identifies the institute. The BIPM publishes monthly results that allow laboratories to compare their UTC(k) realizations with UTC. This is how one coordinated reference can be disseminated through many institutions without relying on one physical clock.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsUTC, TAI and UT1: three related time scales
| Scale | What it represents | How it relates to UTC |
|---|---|---|
| UTC | The international reference time scale used for coordinated time signals and as the basis for civil time. | Computed by the BIPM; historically adjusted with leap seconds to stay approximately aligned with Earth-rotation time. |
| TAI | International Atomic Time, a continuous atomic time scale computed by the BIPM from contributed clock and time-transfer data. | Runs at the same rate as UTC and differs from it by an integral number of seconds. |
| UT1 | A time scale that reflects Earth’s rotation. | Earth-rotation measurements inform the leap-second adjustments that have historically kept UTC close to UT1. |
The distinction is useful: TAI is continuous atomic time, UT1 reflects Earth’s rotation, and UTC is the coordinated reference scale that has historically bridged the two. UTC(k) is not another global time scale; it is a laboratory’s real-time local realization of UTC. BIPM: Time metrology
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What defines the second?
The SI second is defined by fixing the caesium-133 unperturbed ground-state hyperfine transition frequency at exactly 9 192 631 770 Hz. In other words, one second corresponds to the duration of 9 192 631 770 periods of the specified radiation. This definition supplies the unit used in precision timekeeping; it does not mean UTC is the reading of one caesium clock. BIPM: The second
Why UTC has leap seconds
Earth’s rotation does not provide the same steady rate as atomic time. Historically, leap seconds have been inserted into UTC to keep its difference from UT1, the Earth-rotation scale, within an approximate range. A leap second changes UTC’s accumulated offset from TAI; it does not change the length of the SI second.
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The International Earth Rotation and Reference Systems Service (IERS) supplies Earth-rotation information and decides and announces leap-second insertions under the applicable procedures. The 2022 General Conference on Weights and Measures (CGPM) resolution recounts the procedure in ITU-R Recommendation TF.460-6: an adjustment is applied when the predicted difference UT1−UTC approaches 0.9 seconds. CGPM Resolution 4 (2022) IERS: International Celestial Reference System
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The formal definition of UTC and TAI was adopted by the CGPM in 2018. That institutional definition preserves UTC’s role as the reference scale while allowing the method of maintaining its relationship to Earth rotation to be governed through international procedures. BIPM: Time metrology
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How UTC differs from GMT
UTC and GMT are often used interchangeably in everyday conversation to mean the zero-offset reference for time, but they are not universally identical in technical, historical, legal, and software contexts. The available official sources establish UTC’s definition and production, but do not settle every GMT usage. When precision matters, use UTC for the international reference time scale and specify the intended convention rather than assuming the labels are interchangeable.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where people encounter UTC
Most people do not read UTC directly from a special clock. They encounter it through local civil time, time signals, or synchronized network systems. The BIPM identifies navigation systems, telecommunications, financial transactions, scientific facilities, and digital infrastructure among the applications that depend on coordinated time. BIPM: Time metrology
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- Easy to See: Designed with big numerals by AKCISOT.
- Keep Time Accurate: AKCISOT clock movements undergo thorough testing before being offered for sale.
- Silent: Features AKCISOT's quiet sweep movement.
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- What You Get: An AKCISOT wall clock and two hooks.
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- Easy to See: Designed with big numerals by AKCISOT.
- Keep Time Accurate: AKCISOT clock movements undergo thorough testing before being offered for sale.
- Silent: Features AKCISOT's quiet sweep movement.
- Simple Design: Presents a clean clock face by AKCISOT.
- What You Get: An AKCISOT wall clock and two hooks.
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