Craig Colvin has “always been fascinated by unusual clocks,” and his magnetic number build certainly meets that requirement. According to his write-up, magnetic numbers are placed on a piece of thin white plexiglass with a sheet of metal laminated to the back, which allows them to normally stay in place.
On the back of this assembly, a CoreXY mechanism is implemented, which is typically used to move a print head in a 3D printer. Here, however, the carriage contains a pair of magnets that are engaged to capture the magnetic numbers and slide them around. When in the proper position, the magnets disengage and they stay in place as you would expect on a refrigerator, whiteboard, or similar.
The project is demonstrated in the first video below, changing the time on display. It looks pretty magical, or like someone is manipulating it from the back. The second clip, though, shows how it works, with the stepper-powered CoreXY assembly taking care of this manipulation duty. It's controlled by an Arduino-compatible, SAM D21-based board from RobotDyn (SAMD21 M0-Mini), along with a PCF8523 RTC unit for timing. Steppers are directly powered by a CNC shield with A4988 motor drivers. Code for the clock can be found on GitHub.