Radio controlled watches work by picking up a radio transmission from one of 6 points around the world to update the time with an atomic clock.
What's the advantage of a radio controlled watch?
So the main question is - why? Well, once the watch has been set to the relevant time zone, they always display the exact local time. How good is that?
Regular signal reception ensures that your watch will always show the correct time. The exact time is transmitted to the watch from atomic clocks which are practically the most precise watches in the world. Basically, No setting, no resetting, and always on time.
How accurate are atomic clocks?
Atomic clocks are by far the most accurate we know of, on average, an atomic clock is accurate to around 1 second in 300,000 years. To put this into a better picture: a quartz wristwatch falls a few seconds out of sync each month. A mechanical wristwatch, regardless of how sophisticated they are, lapse many more seconds behind.
The in-depth answer to this is that usually we think of a second as 1/60th of a minute. However, the scientific definition of a second is based on the number of times electrons jump back and forth between energy states around a Cesium-133 atom (a particular isotope of the atom). In 1967, scientists at the General Conference on Weights and Measurements in the U.S.A., used an atomic clock to define a second as “9,192,631,770 periods of the radiation” of a Cesium-133 at absolute zero (a temperature defined as 0 K, or −273.15°C, commonly known as Absolute Cold). Being able to measure that many events in that short an amount of time gives you an inkling how accurate an atomic clock is.
How does a radio controlled watch work?
All radio controlled watches feature a tiny radio transmitter/receiver built somewhere within the mechanism, designed to pick up frequencies at 77.5 kHz (DCF77: Germany); 60 kHz (MSF: UK); 60 kHz (WWVB: USA); 40 kHz (JJY: Fukushima, Japan) / 60 kHz (JJY: Kyushu, Japan), and 68.5 kHz (BPC: China). This picks up the transmission broadcast from Fort Collins in the U.S.A., Anthorn in the U.K., Mainflingen in Germany, Shangqiu in China, Mount Otakadoya and Mount Hagane in Japan to keep the watch in-line with atomic clock parameters.
What happens when I travel?
When travelling internationally, radio controlled watches need to be set to the destination time-zone in order to pick up the relevant time. Once this has been done, the watch performs a "manual receive" to pick up the local broadcast and ensure the correct time is displayed.