Crazi spinning HardDrive Clock from Hacked Gadgets Forum contributor NatureTm.
No code provided (yet?)

“The first interrupt just records the time that the slot reaches the sensor and sets the position to zero. [It actually sets the position to the offset value since the sensor isn’t at 12 o’clock and I like to think of 12 as pos 0.] Now I can tell how long a rotation takes and how long it has been since the last completed rotation.”
Via hackedgadgets (read the original forum post here).