If you tune a shortwave radio to 2.5, 5, 10 or 15 MHz, you can hear a little part of radio history — and the output of some of the most accurate time devices on Earth.

Depending on where you are in the United States, those frequencies will bring you to WWV and WWVH, two extremely accurate time signal stations.

Developed before commercial radio existed, WWV recently celebrated its 100th anniversary. It’s the oldest continually operating radio station in the United States.

Both stations are overseen by the National Institute of Standards and Technology, the federal agency that governs standards for weights and measures and helps define the world’s official time.

That time can be heard on shortwave radio 24/7.

Early on, WWV focused on experimental broadcasts (think Victrola concerts that wowed early radio users). Beginning in 1945, it began broadcasting time, too. In 1948, it was joined by a sister station in Hawaii for better Pacific Coast coverage. Unlike its Pacific counterpart, WWV also broadcasts at 20 MHz.

The format is bare-bones: Ticktock-like tones mark each second. Every minute, a voice announces Coordinated Universal Time, also known as UTC, which corresponds with Greenwich Mean Time. The stations also broadcast marine storm warnings, Department of Defense messages and updates on the status of GPS satellites and solar activity.

The information is provided by cesium atomic clocks and is accurate within less than 0.0001 milliseconds. The signal takes a tiny bit of time to travel to radio listeners, but is accurate within 10 milliseconds in most places within the United States. The stations are most recognizable to shortwave radio fans, and they are used to calibrate stopwatches, synchronize clocks for scientific and industrial applications, and even to tune pianos and time astronomical observations.

But another sister station, WWVB, is less familiar and arguably more widely used.

It doesn’t broadcast voices, just digital time codes over a ­low-frequency carrier. In North America, millions of radio-controlled watches and alarm clocks sync up with WWVB.

Don’t have a shortwave radio, but want to listen in? Call 303-499-7111 for WWV or 808-335-4363 for WWVH, and listen to up to two minutes of the oddly comforting broadcasts.