This is a clever repurposing of an old bike chain and cassette. A synchronous motor, placed behind the cassette, drives it around at one rotation per hour. (The gif above is time-lapsed.)
The Maker, Brian Barmmer, placed 3D printed numbers, from 1 to 12, in the chain at 18-link intervals, equal to the number of teeth on the cog that holds the chain, so that each rotation corresponds to an hour.
The motor turns in sync to the voltage coming from an AC outlet, and so can be geared to the appropriate speed. Barmmer also routed a wooden case to hold the motor, but it’s housed discreetly behind the cassette.
See Barmmer’s imgur gallery for more details on the build.