Julldozer created an amazingly clever digital sundial ("Cadran Solaire Numérique") that precomputes the angle of the sun throughout the day and uses those computations to make hundreds of precise holes calibrated to cast a shadow displaying the present time.
No batteries, no motor, no electronics… It's all just a really super-fancy shadow show. The shape of the sundial has been mathematically designed to only let through the right sunrays at the right time/angle. This allows to display the actual time with sunlit digits inside the sundial's shadow.
The sundial displays time (with actual digits !!) from 10:00 until 16:00, updating every 20 minutes.
You can precisely adjust the displayed time simply by rotating the gnomon (the magic box that displays time). So you can even adjust for Daylight Saving Time.
ep. #001: Cadran Solaire Numérique