Clock

Clocks work like compasses, in the sense that you cannot use them to change the physical world, but their image changes depending on the circumstances. The clock displays the sun and the moon's position relative to the horizon.

Clocks were added in the Halloween Update on October 31, 2010.

Crafting
Clocks are created by using one redstone dust and four gold ingots (see formation below).

Telling the time
Below is a gif showing how the clock works. The clock consists of two halves with a day side and a night side. These spin slowly to tell you the time of day, corresponding to what is at the top most point.



Alternatives
If you do not have a clock, you can approximate the time by looking at the position of the Sun or Moon in the sky. When underground, or in an area without a view of the sky, you can use the sounds of nearby enemies as a cue to the current time; if you hear the sounds of enemies taking damage, you can assume it's dawn and they are on fire. If you are very deep underground, try to find a spot where you can see the void; the color of it corresponds to day and night.

In the menu, the item statistics list shows an accurate clock next to your clock statistics.

Furthermore, one quick trick to tell the current time while underground is to toggle the view distance (F key). Doing this redraws all blocks around you, which may give you just enough time to see if the sun or moon is out. (This no longer works in Beta 1.8)

You may also try mining a long tunnel, of 2 blocks high and 1 block wide, and looking down it from a distance with the shortest view distance selected (Press F). You should notice how the lighting in the hallway will become a lighter whitish color during the daytime, and will be a darker blue hue during the night. This technique will work at any level in the earth.

You can also (if you have the mod TooManyItems installed) just look at the icon of the clock. (Works too with compasses)

Textures
Unlike most items the clock combines 2 textures, one being the actual clock, and the other, the dial. So you may change the clock's texture without worrying about breaking the dial, as long as you do not change the transparency.