To read the system time from a Digital TV transmission, the tuner card installed in your machine must firstly be tuned to a channel. Then execute this command in a terminal.
localhost% dvbdate Mon Apr 17 01:15:33 2017
This will read the time from the digital TV transmission and print it to the terminal.
Very nice Linux trick.
This does not work unless the digital TV tuner is already tuned, but it is very good trick though.