## Saturday 16 July 2016

### Blok Clok

When I was a boy, I was given a curious present: a perpetual desk calendar made from wooden cubes:
You can still buy them, so there must be some enduring appeal. The calendar interested me because of a strange quirk that isn't immediately obvious.

There are four blocks: one for the day of the week (with seven labels), two for the day of the month (1 to 31) and one for the month (with twelve labels). The "weekday" and "month" blocks are easy to label. For example, two months on each face fills the "month" block and, providing the container covers up the lower half of the block, all is peachy.

The two date blocks are a little more involved. Dates run from "01" to "31":

01,02,03,04,05,06,07,08,09,10,
11,12,13,14,15,16,17,18,19,20,
21,22,23,24,25,26,27,28,29,30,31

By inspection, we see that the only digits that appear twice on a single date are "1" and "2". However, if you sit down with a pencil and paper, it quickly becomes apparent that you'll also need two zeroes. That means that the following digits are required:

0,0,1,1,2,2,3,4,5,6,7,8,9

But, hang on! That's thirteen digits. Two cubes have only twelve faces between them. How do they fit? The clever "quirk" is that "6" and "9" occupy the same face; you simply flip the block around 180 degrees. This gives you the following faces for the two blocks:

0,1,2,3,4,5
0,1,2,6,7,8

This arrangement allows to you represent all integers between "00" to "32" inclusive; just enough! It's deceptively clever. And that's probably why I like my wooden perpetual calendar so much.

Fast forward several years, if not decades, and my current fascination with clocks led me to consider whether you could tell the time with a similar arrangement. I knew representing the hours would not be a problem: as we've seen, two blocks can represent all integers between "00" to "23" inclusive for a twenty-four-hour clock. But representing minutes is harder.

It is fairly obvious that you need at least three blocks to represent all integers between "00" to "59" inclusive. But are three sufficient? It turns out that they are; you don't even need to use the upside-down "9" trick. The solution uses an additional cube to display a colon between the hours and minutes digits. For example:

23:59

The "tens minutes" digit takes up a single block: "0" to "5" inclusive. So the problem simplifies to trying to represent the "units minutes" digit ("0" to "9") and a colon with two blocks. The following labelling is one solution:

:,0,1,2,3,4
:,5,6,7,8,9

Using three blocks to represent minutes between ":00" to ":59" inclusive can be repeated for the seconds. This gives us eight blocks:

hours-a    0,1,2,3,4,5
hours-b    0,1,2,6,7,8
minutes-a    :,0,1,2,3,4
minutes-b    0,1,2,3,4,5
minutes-c    :,5,6,7,8,9
seconds-a    :,0,1,2,3,4
seconds-b    0,1,2,3,4,5
seconds-c    :,5,6,7,8,9

Not only do the blocks need to rotate to any of their six faces (and in the case of "hours-b", the "6" sometimes has to be flipped to become a "9") but pairs of blocks also have to be swapped periodically:

hours-a <=> hours-b
minutes-a <=> minutes-c
seconds-a <=> seconds-c

Some extra fettling is needed to make the rotations aesthetically pleasing, but the result is a twenty-four-hour clock that twists and turns at a fair old lick.