You might have the most expensive watch in the world, but if it’s set to the wrong time to begin with, it’s no use to you at all. Even really good quartz clocks struggle to keep time to better than a second a day; if they wander out by just a couple of seconds in 24 hours, and the errors don’t cancel out, that could add up to a minute a month or almost a quarter of an hour a year. That’s why most people regularly check their watches against a reliable time signal—like the ones you hear before news broadcasts on radio stations. Now wouldn’t it be cool if your watch could listen to those broadcasts and set itself to the right time automatically without you ever needing to worry? That’s the basic idea behind radio-controlled clocks and watches, which set their time by super-accurate atomic clocks. Let’s take a closer look at what these things are and how they work!

What is an RCC?

An ordinary clock or watch is a time-counting device that adds up the number of seconds, minutes, hours, and days that have passed. But it doesn’t actually know what time it is until you tell it: it’s not a time-keeping device unless you set it to the right time to start with. A radio-controlled clock (RCC) is different. It’s similar to an ordinary electronic clock or watch but it has two extra components: an antenna that picks up radio signals and a circuit that decodes them. The circuit uses the radio signals to figure out the correct time and adjusts the time displayed by the clock or watch accordingly. Unlike an ordinary clock or watch, an RCC always knows what time it is—you never have to tell it!

The radio signals come from a unique radio “station” that doesn’t broadcast any words or music. There’s no DJ and no irritating advertisements for car insurance. All the station broadcasts is the time—over and over again—in the form of a special code that only radio-controlled clocks can understand. In the United States, these time signals are broadcast by a station called WWVB operated by the National Institute of Standards and Technology (NIST) from a base near Fort Collins, Colorado. The NIST time code contains the basic time and date, whether it’s a leap-year, whether it’s daylight-saving time, and so on and takes about a minute to broadcast in its entirety.

Most Radio controlled clocks synchronize themselves with a time broadcast signal once a day, at night, although some check themselves every few hours. Generally, that gives them an accuracy of better than plus or minus a half second a day. Another advantage is that they automatically correct themselves for daylight-saving time, leap years, months with different numbers of days, and so on.

It’s pretty obvious that a radio controlled clock is only going to be as accurate as the time signals it uses to regulate itself. But how can you be sure those are accurate? The time-signal radio stations operated in different countries broadcast UTC (Coordinated Universal Time), the officially agreed time used worldwide that’s informally known as GMT (Greenwich Mean Time). UTC is maintained by hundreds of atomic clocks (which are the world’s most accurate timekeeping devices) around the world, all of which are synchronized with one another. It’s because RCC radio signals are based on time kept by atomic clocks that you’ll sometimes see Radio controlled cock manufacturers describing their products as “atomic” clocks and watches (even though they’re really no such thing).