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Coordinated Universal Time (UTC) is the international reference time scale used to coordinate standard frequencies and time signals around the world. The Bureau International des Poids et Mesures (BIPM) computes UTC from data contributed by laboratories; it is not the reading of one master clock. UTC runs at the same rate as International Atomic Time (TAI), while leap seconds have historically adjusted its offset from TAI to keep it in approximate agreement with time based on Earth’s rotation.
What does UTC mean?
UTC stands for Coordinated Universal Time. It is the shared reference used to distribute standard frequencies and time signals, and it provides the basis for coordinated civil timekeeping. Navigation, telecommunications, financial transactions, scientific facilities, and digital infrastructure all depend on coordinated time.
The formal definition of UTC and TAI was adopted by the General Conference on Weights and Measures (CGPM) in 2018. In practice, UTC is computed internationally from measurements and comparisons among many clocks, rather than being generated by a single clock.
How is UTC produced?
The BIPM computes UTC from clock and time-transfer data contributed by laboratories around the world. Its current UTC page, accessed in 2026, reports contributions from around 80 time laboratories and approximately 450 atomic clocks. These are approximate counts reported by BIPM, not a count of clocks that independently display the official time.
#1 Best Overall
- Atomic Wall Clock Never Need Setting: Atomic clock has a built-in receiver that automatically synchronizes itself with the WWVB radio broadcast by the US Government's National Institute (NIST) in Fort Collins, Colorado. The Atomic Clock will always be accurate to within one second as it receives daily WWVB updates. (No Backlight)
- Atomic Clock with Indoor/Outdoor Temp: The Wireless outdoor sensor transmits the outdoor temperature to the atomic clock. The sensor should be placed within 100ft of the clock. Also, clock has a built-in high-quality indoor temperature sensor so that you can read the temperature both indoor and outdoor
- Digital Wall Clock Large Display: Jumbo 2.5 inch Height Time Digital Display - Use as either a wall clock or as a desk clock, it is easy to read, easy to Use and easy to set up. This atomic clock has been designed with absolute simplicity in mind, you can also set it manually. Excellent clocks for seniors, suitable for anyplace in your home or office
- Atomic Clock Digital: 12/24 Hour time formats with accurate and clear second minute and hour display; 4 time zone to choose (PST, MST, CST, EST); auto daylight saving time; temperature display units in Celsius or Fahrenheit
- Digital Wall Clock Battery Operated: Both clock and outdoor sensor are supported by batteries only, no need to plug or charge (Batteries not included). The existed alarm features can wake you up in the morning or help you keep track of dates and days of the week to avoid missing any important meetings or appointments
Each participating laboratory maintains a real-time local realization called UTC(k), where “k” identifies the institute. The BIPM publishes monthly results that allow laboratories to compare their UTC(k) realization with UTC. UTC(k) is therefore a laboratory’s practical realization of the international scale, not a separate global standard.
UTC, TAI, and UT1: what is the difference?
| Time scale | What it represents | How it relates to UTC |
|---|---|---|
| UTC | The international reference time scale used to coordinate time signals and civil time. | Computed by BIPM from contributed clock and time-transfer data. Its rate matches TAI; its offset from TAI has historically changed by whole seconds. |
| TAI | Continuous atomic time. | Computed by BIPM from contributed data. UTC differs from TAI by an integral number of seconds. |
| UT1 | Time derived from Earth’s rotation. | Earth-rotation information is used to maintain UTC’s approximate agreement with UT1. |
The SI second underpins precision timekeeping. It is defined by fixing the caesium-133 unperturbed ground-state hyperfine transition frequency at exactly 9 192 631 770 Hz. That definition establishes the unit of time; it does not mean UTC is the output of one caesium clock.
Rank #2
- Atomic Clock Digital - The clock has a built-in receiver that automatically synchronizes itself with WWVB radio broadcast, which will set and update time automatically every midnight, no need to set manually. It has four time zones selectable: EST, PST, MST, CST.
- Digital Wall Clock Battery Operated - This digital clock is supported by 3xAAA battery (Not included). No wires make it can be placed anywhere; and the low battery symbol reminds you to replace the battery in time to avoid getting the wrong time due to low power.
- Digital Wall Clock Large Display - The LCD screen measures 6.2 in x 3.7 in and it displays time, indoor temperature, day and date. Each section is showed separately, allows you to read the content easily and quickly.
- Atomic Clock with Auto DST - With DST on, the time would go forward or backward one hour automatically in daylight saving time, no worry about time changing on that day. If for no need, it could also be turned off, then “AUTO DST” disappears from screen.
- Digital Alarm Clock - The alarm could be turned on or off according to your need, just click the button to set. When alarm is set on, there is a bell icon staying on screen. The icon will disappear if alarm is set off. When alarm goes off, just click any button to stop it on the day.
Why are leap seconds used?
Earth’s rotation is not the basis for the SI second, and time based on Earth’s rotation can drift relative to a continuous atomic scale. UTC has historically used leap seconds to keep its relationship with UT1—Earth-rotation time—within an approximate range. A leap second changes UTC’s accumulated offset from TAI; it does not change the duration 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. CGPM Resolution 4 of 2022 recounts the procedure in ITU-R Recommendation TF.460-6: a leap second is applied when the observed UT1−UTC difference is predicted to approach 0.9 seconds. That threshold describes the procedure, not an announcement that a leap second will occur on a particular future date. For an upcoming insertion, rely on current IERS notices.
Rank #3
- Atomic Digital Clock Auto Set (No back light) - Using radio frequencies broadcast from NIST’s Colorado , the clock will automatically set to the correct time. Automatically adjusts to Daylight Savings Time while set the DST on.
- Atomic Wall Clocks Battery Operated with Alarm - This digital calendar alarm day clock required 3 “AA” long life batteries (Not included) for operation, no redundant wires, easy to use. Turn on the alarm clock before going to bed, it will wake you up in the morning.
- Clock with Temperature and Humidity - Excellent wall clocks for living room, office, bathroom decor,etc. Help you monitor indoor temperature and humidity to avoid cold. 12/24 hours formats with clear Hours, Minutes and Seconds display; 4 Time Zone to choose.
- Atomic Clock Large Display for seniors - WallarGe atomic clock has been designed with absolute simplicity in mind, you can also set it manually without any more ado. Wall clock or desk clock, large and clear digital display, easy to read.
- Digital Wall Clock with Accurate Date and Time - This digital calendar alarm day clock can help you keep track of dates, days of the week and times to avoid missing any important meetings, appointments or anniversaries, work well for work, study or travel schedule.
Is UTC the same as GMT?
UTC and GMT are often used interchangeably in everyday conversation to refer to the time standard at zero offset, but they are not automatically identical in every technical, historical, legal, or software context. UTC is the internationally coordinated time scale described here. The sources cited for this definition do not establish every distinction in how GMT is used, so avoid treating the terms as universal technical synonyms without specifying the context.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where do people encounter UTC?
Most people encounter UTC indirectly: as the reference behind local civil time, time signals, or synchronized network systems. Devices and services can display local time while relying on UTC-based coordination behind the scenes. UTC matters because it lets systems in different places compare timestamps and coordinate events against a common reference.
Quick Recap
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- [Automatic Time Updates] – The Atomic Time Feature conveniently allows the clock to sync with the WWVB radio transmission from Fort Collins, CO, for accurate time updates.
- [Classic design] – With unmatched attention to detail, the WT-3129B is highlighted by a distinctive matted 12-inch black frame with a silver bezel – a statement piece for any home setting.
- [5-Year Battery Life] – A quality set of 3 “AA” batteries will keep the clock running for up to 5 years – an impressive feat offering unparalleled convenience and peace of mind.
- [SIMPLE-SET] Simple adjustable switch to your Time Zone (4 different selections: PT-Pacific, MT-Mountain, CT-Central, ET-Eastern)
- [A Brand You Can Trust] – Family-owned and operated since 1983, La Crosse Technology is the world leader in Atomic Time and Weather Instruments. Designed in La Crosse, WI, our products have unmatched support resources, including printed setup guides, helpful video tutorials, and lifetime access to our USA-based tech support team.
Rank #4
- Atomic Clock with Backlight: Never Need Setting, a built-in receiver that automatically synchronizes itself with the WWVB radio broadcast, it will always be accurate to within one second as it receives daily WWVB updates. Three levels (OFF-LOW-HIGH) backlight helps you to know what time it is without turning the light on late at night
- Large Digital Wall Clock: 14 inches large display with 4.3 inches numbers make this digital wall clock easy to read and it's visible around the entire room, friendly to the elderly and the visually impaired
- Digital Wall Clock Battery Operated: The clock is supported by four AA battery (not included). No wires make it can be placed anywhere; and the low battery symbol reminds you to replace the battery in time to avoid getting the wrong time due to low power
- Digital Clock with Date and Day of the Week: This digital wall clock with backlight not only does it give you the time, you can also get the day, date and current indoor temp at just a glance. And the day and temp display could be chosen to display alone or in turns every 10s
- Multiple Display Formats: Large digital wall clock with backlight makes it easy to know the time, two different time and temperature formats (12/24H; /) are available for your convenience and preference
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