Clock

Clocks (sometimes called watches), are items that display the current in-game time by displaying the sun and the moon's position relative to the horizon.

Telling the time
The clock consists of two halves, a day side and a night side. These spin slowly to indicate the time of day, corresponding to the sun or moon's actual position in the sky. This allows the player to tell the times of dawn, noon, dusk and midnight. The player is able to sleep a few seconds after the clock shows exactly dusk (when the line between day and night on the dial is straight). This can be useful for getting to sleep as soon as possible, therefore not allowing any hostile mobs to spawn in the darkness. A clock placed in an item frame will continue to show the correct time, effectively making a "wall clock".

The clock can be useful in caves, as it will let the player know when they can sleep in the cave, and when it is safe to leave. Looking at a clock is sometimes quicker than looking into the sky, and is still just as useful in caves. Just like compasses, clocks do not work in the Nether or The End, their dials will only rewind rapidly showing random time of days, making it useless for determining the position of the sun and the moon.

Alternatives
If the player does not have a clock, they can approximate the time by looking at the position of the sun or moon in the sky. In the menu, the item statistics list shows an accurate clock next to your clock statistics. Placing the clock ingredients on a crafting table will give an accurate clock icon, even if the player does not finish crafting the clock.

Furthermore, one quick trick to tell the current time while underground is to toggle the view distance in the options menu. Doing this redraws all blocks around the player, which may allow the player just enough time to see if the sun or moon is out (works better on slower computers).

You may be able to use the Bed Behavior to see if it is day or night. Right clicking on the bed in daytime will yield the "You can only sleep at night" message. This isn't always accurate because it is possible to sleep during thunderstorms during daytime.

Trivia

 * A clock will function whether it is in the player's inventory, in a chest, about to be crafted, on the ground, in the statistics menu, in someone else's hand, in a frame, or anywhere else, even if it is not supposed to be there.
 * If sleeping with a clock in your hand, upon waking, or the time is changed through the commands, the clock dial will spin rapidly to catch up.
 * After completing 100 day-and-night cycles in the Xbox 360 release of Minecraft you are awarded a miniature clock wristwatch for your Xbox Live avatar to wear.
 * In the clock texture, the dimensions in pixels are 16x1024, confirming that the entire clock animation consists of 64 different pictures.

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