I spend some time today to generate my own signal using this method:
dcf77 , wich is (for the non german speakers) basicly the idea to let a linux machine generate a black/white pattern on the screen. This pattern let the Monitor generate an electromagnetic wave which equals the 77.5 kHz carrier wave for the time signal and is then mudlated to transmit the time information.
After installing some packeges to get the code compiled and setting the xserver to a lower color depth (16bits) I could generate the patterin in a high enought framerate.
For testing I used a common radio controlled alarmclock, which gets the official time signal very well. The signal from the littlt dcf77 programm starts sending ttime at a fake value of 23:23, so success is easily recognizable (unless you run the test at 23:"3, of course). But you can run anny starting time with parameters.
So far I wasn't able to set the clock to my own time with this method. ONly "success" was that I could prevent the clock to get the official time signal while in the range of the monitor, which was sending the fake time.