Martin Stromer made a great looking Hard Disk Clock that is actually very easy to read and looks like a great art piece at the same time. It is based around the ATMega16 Microcontroller.
“The time is set by turning the disk and setting the read/write head to the desired time by hand. Then you push the reset button on the back (the upper one of the tree on the back) and the clock measures the position of the indicators and sets its internal electronic quartz clock accordingly. For measuring the hour I painted a binary pattern onto the backside of the disk which is illuminated by infrared LEDs and detected by matching infrared light sensors.
For detecting the set minute the read/write head is driven to the outside of the disk until it interrupts the a light barrier. The steps necessary for that are counted, the internal clock is set and the head is driven back to its previous position.
Because the current time is stored in an internally the time display can be restored if it was changed by accident. This does the middle button on the back. Pushing it will turn the disk and move the head to the correct position.”
 
