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In this article 7 sections
"Atomic clock" means two different things. There are real atomic clocks: laboratory instruments that count the vibrations of atoms and keep the official time. And there is the "atomic clock" on your kitchen wall, which has no atoms ticking inside at all; it listens to a radio signal from those laboratory clocks. This page explains both, with the National Institute of Standards and Technology (NIST), which runs the U.S. time signal.
Short answer: a real atomic clock tunes microwaves to the exact frequency at which cesium atoms change energy state; NIST says the second is defined as 9,192,631,770 cycles of that frequency. A home "atomic" clock is really a radio controlled clock: it receives the time from NIST radio station WWVB in Fort Collins, Colorado, which sends a time code on 60 kHz at 1 bit per second. The clock decodes it, usually once a night, and corrects itself.
Editorial Research
- Written by
- Lena Thornton, Weather Station Analyst & CWOP Contributor
- Based on
- NIST: "How do you measure a second?", Radio Station WWVB, and Special Publication 960-14 on WWVB radio controlled clocks (2009)
- Research last verified
- October 29, 2026
How a Real Atomic Clock Works
According to NIST, the international unit of time, the second, is defined as the duration of 9,192,631,770 cycles of the radiation of a cesium atom. A cesium clock aims microwaves at cesium atoms; at exactly the right frequency the atoms absorb the energy and make a "quantum jump" to a different energy state. The clock is tuned to the frequency at which the most atoms make that jump, and counting those cycles gives the second.
How good is that? NIST says the best cesium clocks are accurate to about one three-hundred-millionth of a second per year, and that the JILA strontium optical lattice clock, run jointly by NIST and the University of Colorado Boulder, would not gain or lose a second in 15 billion years. Your oven clock, by comparison, is not playing in the same league (our joke, not a NIST figure).
The "Atomic" Clock on Your Wall Is a Radio Controlled Clock
NIST itself is clear about the name. In its Special Publication 960-14 it calls the term "atomic clock" for home clocks "technically incorrect and misleading to consumers", because there is no atomic oscillator inside, and recommends "radio controlled clock" instead. What the clock does have is a small receiver permanently tuned to WWVB.
| WWVB fact | According to NIST |
|---|---|
| Location | Near Fort Collins, Colorado, about 100 km (60 miles) north of Denver |
| Frequency | 60 kHz, a low frequency; even the bottom of the AM band (530 kHz) is nearly nine times higher |
| Power | Effective radiated power of 70 kW |
| Coverage | The continental U.S.; at night also parts of Alaska and Hawaii, Mexico, most populated parts of Canada and some regions of Central and South America |
| Broadcast | 24 hours a day, 7 days a week, a complete time code every minute |
| Source of the time | Groups of atomic oscillators at NIST, compared with a second time scale at the station; the difference is nearly always less than 20 nanoseconds |
Sources: NIST Radio Station WWVB page and NIST SP 960-14 (2009 edition).
How the Time Code Works
NIST describes the WWVB time code like this: 60 bits of information, sent at 1 bit per second, so a full time code takes one minute. Every second, the station lowers the power of its 60 kHz signal, and the length of that dip carries the information:
- 200 milliseconds low = a 0 bit
- 500 milliseconds low = a 1 bit
- 800 milliseconds low = a frame marker, sent every 10 seconds
The bits spell out the minute, hour, day and year, plus notices for daylight saving time and leap seconds. In 2012 NIST added a phase-modulated signal on top; NIST says existing radio controlled clocks were not affected and continue to work as before.
Time Zones, Daylight Saving Time and Leap Seconds
- Time zone: WWVB broadcasts UTC, not local time, so NIST says every clock needs a time zone setting to show local time.
- Daylight saving time: the time code carries DST information, and NIST recommends that clocks use it rather than a built-in rule, because the code always follows the current rules (the rules changed in 1986 and 2007). Clocks should switch at 2 a.m. local time on the second Sunday of March and the first Sunday of November. Where DST is not observed, NIST says clocks must let you switch DST off.
- Leap seconds: when needed, typically less than once a year, a leap second is added on June 30 or December 31. The time code has a bit that announces it, and the last minute of that day then has 61 seconds.
When Does Your Clock Sync?
NIST says radio controlled clocks usually read the time code only periodically, often once every 24 hours, typically at night when the signal is strongest. It recommends that clocks keep time to within ±0.5 seconds of NIST time between synchronizations. La Crosse clocks, for example, search at set hours overnight; our page how to set a La Crosse atomic clock lists the times and buttons per model.
Clock not syncing? NIST's advice is mostly about placement: away from TVs, computer monitors and appliances with motors, near a window in metal buildings, not in a basement. Our page on atomic wall clocks with temperature has NIST's full list.
Atomic Clock FAQ
How does an atomic clock work?
A real atomic clock tunes microwaves to the frequency at which cesium atoms make a quantum jump between energy states and counts those cycles. NIST says the second is defined as 9,192,631,770 cycles.
Is my atomic wall clock really atomic?
No. It is a radio controlled clock that receives the time from NIST radio station WWVB, whose time comes from atomic clocks. NIST calls the "atomic clock" label for these products technically incorrect.
Where does the U.S. atomic clock signal come from?
From NIST radio station WWVB near Fort Collins, Colorado, broadcasting on 60 kHz around the clock.
How often does an atomic clock update?
Usually once a day, at night when the WWVB signal is strongest, according to NIST, which also recommends several attempts per night.
How accurate is an atomic clock?
NIST says the best cesium clocks are accurate to about one three-hundred-millionth of a second per year. A radio controlled home clock should stay within ±0.5 seconds of NIST time between syncs, NIST recommends.
