This mechanism uses 7 pairs of inverse cams to push and pull on a rack and pinion. The pinions are connected to the segments that make up the numbers on the front of the display. As the pinions flip up and down, the number changes according to the positions of the cams.
This is a fully 3d printed model, which you can get from here, that has an incredibly smooth and satisfying motion. I have also included an option to attach a servo to the back of it, so you can print 4 of them and easily create a clock!
Check out this video to see how easy this thing is to assemble!