Over on YouTube, Jeff Geerling has uploaded an interesting video showing how RF-based atomic clocks can be set via signals generated from a computer speaker. In the USA, RF-based atomic clocks typically receive their atomic time signal from the WWVB 60 kHz longwave radio station, operated near Fort Collins, Colorado. In other countries, different time signal transmitters operate on different frequencies. However, these time signals cannot be received everywhere due to interference or geographic limitations, making RF atomic clocks useless in these situations. 

As Jeff points out, a Time Station Emulator program can be used to locally emulate the WWVB or other time signals, which, while not providing atomic time accuracy, could still make these clocks useful again.

Most interestingly, the emulator program requires no special RF transmission hardware. Instead, it simply uses your computer speakers to broadcast the time signal.

By carefully crafting a waveform at a specific audio frequency (out of normal human hearing range), the digital-to-analog converter will generate higher frequency RF harmonics, and one harmonic will match the time signal frequency required by the RF-based atomic clock. The wires running to the speakers, and the speakers themselves, will act as antennas, leaking these harmonics into the surrounding environment. This means that cheaper unshielded speakers, such as those found in phones and tablets, tend to work better.  

In the video, Jeff uses a HydraSDR and an upconverter to receive the time signal generated by the speakers. While the time signal cannot be seen on the spectrum itself, in the demodulated audio, you can hear the signal's pulses.