On Screen Clocks

How do they work? Is it like a video file that changes every minute, or what?

Used to be done by pointing a camera at a clock, then in the 80s, solid state electronic devices (see http://www.bbceng.info/Designs/designs_reminiscences/Richard_Russell/rtrdd.html), and later computers.

GNAT.

What about the digital ones?

Done by computer usually. No different from the one on your PC screen, in principle.

What software is used to create it?

MS Paint and a dwarf in a box...

Similar things can be achieved in Flash...

This thread feels like it's in entirely the wrong place...