Cheap Perpetual Calendar

At first glance, this is about as sensical as one of those crazy watches from Japan.

Looking a little more closely, you can start to see the pattern. There are three columns with three, four, and five pennies in them. In the three columns, we use heads or tails to spell out a binary number for the day of week, month, and date (day of the month).

(Aside: if you are into this sort of calendar, you might like some of these other interesting designs.)

Here's a concrete example for the date this article was written, Wednesday, December 31, 2008:

It's Wednesday, the fourth day of the week. The binary number for 4 is 100. We use "heads" for 1 and "tails" for 0. So the left hand column has (top to bottom) heads-tails-tails = 100 binary = 4th day of week = Wednesday. (If you don't already speak binary, no biggie. Start here or here and join us in a minute.)

It's December, month 12, and 12 in binary is 1100, so the middle column is heads-heads-tails-tails.

Finally, the last column is all heads, since it's December 31, and 31 decimal = 11111 binary.

You can, of course, gussy it up a bit beyond the cardboard. ("Be extravagant-- use dimes!")

We built a fancy desktop version of this from laser-cut acrylic, with recessed areas to contain the pennies and cutaways so that you can tip them out easily:

Finally, we wrote an applet to help you get started; it displays today's calendar, based on your system clock. It has glacial but live updating-- once per second, and only if the date has changed. If it looks the same as the picture above, you must be viewing this on Wednesday December 31.

(Embedded Java applet follows; you won't see it if your browser doesn't have Java installed.)

The applet is written in Processing; you can download the source code here (16 kB .ZIP file).