On July 3, 1957, John Stephenson Singleton filed for a patent with the UK Patent Office. His invention was called “Improvements in and relating to perpetual calendar devices,” and described a way by which two cubes could be used to display all the days in a month.
If you’re thirty or older, you may remember these calendars from the bank. There was typically a barrier at the back of the check writing station, with three wells on the top of it and three windows on the side facing the patron. The first of the three wells was rectangular and the remaining wells were square. The bank employee could drop a wooden block into first slot and two wooden cubes into the second and third. The block bore the name of the month; each side of the cubes showed a digit; between the three of them, they could display the current date, e.g., [April].
Mr. Singleton received his patent on March 17, 1958. But I want you to consider something.
One of the criteria for a patent is that the invention be “non-obvious.” On the face of it, Mr.Singleton “improvements in and relating to perpetual calendar devices” seems like a no-brainer: you have three blocks (each with the names of four months on their rectangular-sides, and their square-sides blank) and two cubes with the digits distributed amongst them in such a way that every possible day from 01 to 31 can be shown — what’s so innovative about that. In truth, that final bit — the part about distributing the digits amongst a pair of cubes such that every possible day can be displayed using only the two of them — is considerably more “non-obvious” than it seems. Can you figure out how to do it?
The patent can be seen here — but viewing it (or the comments to this post) will ruin the fun of trying to solve the puzzle. Wait until you’re stumped or, better yet, confident that you have sussed out the answer — you’ll be glad you did.