Clocks were originally added in Minecraft's Halloween Update. They are able to tell you the time of day (day or night) and can be used underground. They are especially useful in caves, allowing players to determine the time of day it is outside.
Similar to the compass, the clock does not work in the Nether. Instead, the day/night halves will spin wildly in the clock, it is determined that this occurs because there is no current day/night cycle in the Nether. The Clock additionally behaves as it does in the Nether, in The End.