The next step: Enhance design so clock can change its own marker.

Or feed the marker's liquid reservoir from a larger tank.

Or use a chalkboard.

Chalk would still run out and need to be replaced. Some kind of sand drawing would be more of an infinite "ink".

Hilariously, a good way to do this with infinite "ink" would be to draw the letters on a touch screen with a stylus.

Or, just use one of the children's magnetic drawing board?

http://en.wikipedia.org/wiki/Magna_Doodle

Or some gears and a battery.