Binary

Three rows of six glass beads on a dark panel. Each row is a number in binary: hours on top, minutes in the middle, seconds at the bottom. A red bead is a one, a grey bead is a zero, and the leftmost bead is worth 32, the next 16, then 8, 4, 2 and 1.

Reading it takes practice for a day or two and becomes a habit after that. The time in plain digits sits faintly in the corner for those who have not got there yet.

If you prefer the same idea in a steel case, try Binardy.