Each cube must bear a 0, 1, and 2. This leaves only six faces for the remaining seven digits, but fortunately the same face can be used for 6 and 9, depending on how the cube is turned. The illustration shows 3, 4, 5 on the right (red) cube, and therefore its hidden faces must be 0, 1, and 2. On the left (blue) cube one can see 1 and 2, and so its hidden faces must be 0, 6 (or 9), 7, and 8.

John S. Singleton from England had patented the two-cube calendar in 1957/8 (British patent number 831572), but allowed the patent to lapse in 1965.