DCF77 is a long wave time keeping signal transmitting at 77.5 kHz from Frankfurt, Germany. It has been active since 1 January 1959. Using simple amplitude modulation, the signal encodes the current time and date, which is used by some devices like railway station clocks in Germany. Because it's a long wave signal transmitting at 50kW, it's possible to receive the signal across Europe, and sometimes even further away if propagation conditions are good.
Recently a DCF77 receiver and decoder program based on Python and GNU Radio has been uploaded to GitHub by henningM1r. It includes a simulator written in Python so you can simulate your own DCF77 signal for testing the receiver too.
Currently the decoder has been tested to work with an Airspy HF+ Discovery SDR, but it should work with any SDR capable of receiver 77.5 kHz if the GNU Radio source block is changed out.