This amazing creation was all over the internet a few weeks ago. I neglected to post about it. I feel that I should do so in case you missed it and because it is possibly the most incredible mechanical device that I have seen anyone make in recent history.
The project, called Plock, was created by art student Suzuki Kango as his senior thesis project. It contains more than 400 wooden parts that control four arms which write out the time on a magnetic drawing pad every minute. The board pushes forward when the time is written, then drops back again. This is a clever way to control the Z-axis. The arms need only be capable of moving in the X and Y axes.
The four large weights suggest that it is weight-driven. There appears to be a balance wheel in the bottom portion, so perhaps that is how it actually tells time. It doesn't seem to be moving at all times however, so this is not clear to me.
The whole thing is made of wood, too! Just astonishing.
There are more images of the writing clock on this web site (text in Japanese).
[ Thanks to Charles and all who suggested I post about this! ]