I spend most of the time out of range, but the technology is simple and robust enough that you can use headphones or a phone speaker as a very, very short-range

I spend most of the time out of range, but the technology is simple and robust enough that you can use headphones or a phone speaker as a very, very short-range transmitter. You hardly need to sync every day, either. I've had most success with https://einoko.github.io/DCF77.js/ https://einoko.github.io/DCF77.js/