I looked into this for a slightly more specific reason, I had a HDD recorder box which had both analogue and DVB tuners, but since the ending of analogue transmissions its internal clock had no longer kept good time. This was because the unit synchronised its time from the analogue teletext only.
My plan, then, was to put together something which would obtain the real time and date somehow, generate a teletext page with the time and date kept updated, and feed that through a synthesised (therefore rock steady) analogue RF modulator and connect the output from that to the HDD box's analogue tuner input.
The idea was that the HDD box would periodically read the time from the analogue teletext signal just as it originally did from analogue BBC1, probably, and keep itself in sync with the real time.
The ideal platform for this was the Raspberry Pi - it can obtain the current time and date through its network / wifi connection, it has the option of composite video out, and there are a few projects which already do this.
Here's one, found at random
https://www.raspberrypi.org/magpi/teefax/
Having said all that, I never did do any of that. Various other factors forced me to buy a new HDD box instead, and of course that gets its time from DVB, or via its internet connection.