Daylight cycle

The day/night cycle is a 20 minute long lapse between two main light settings.

Daytime
Daytime is the longest section of the cycle, lasting 10 minutes.
 * When a player first spawns in Infdev, Alpha, and Beta the cycle will always start right at the beginning of daytime.
 * Daytime features the Sun, and a light blue sky.
 * During daytime, all blocks that are in direct vertical view of the sky receive daylight, the strongest light possible, which is light level 15.
 * Daylight provides ample light for the growth of all trees and crops.
 * Zombies and Skeletons in direct view of the sky (and not in water or ice) during daytime will be set on fire.

Sunset
Lasts 1 ½ minutes.
 * A period between daytime and nighttime, when the Sun sets on the West horizon, and the Moon rises on the East horizon.
 * All blocks that are in direct vertical view of the sky get 1 shade of lighting darker every 10 seconds.
 * The sky near the setting sun will have a red glow when the sun sets.
 * Actual time of Sunset will not change if the texture for the sun is changed. The day/night cycle will be the same length with the same effects even if the sun is visibly larger or smaller.
 * The sun will also get visibly larger when it sets.

Nighttime
Lasts 7 minutes.
 * During nighttime the world is covered in darkness with a light level of 4, and hostile mobs may spawn on the surface.
 * Nighttime features the Moon, and a dark black sky, with white stars.

Sunrise
Lasts 1 ½ minutes.
 * A period between nighttime and daytime, when Moon sets on the West horizon, and the Sun rises on the East horizon.
 * All blocks that are in direct vertical view of the sky get 1 shade of lighting brighter every 10 seconds.
 * The sky near the rising sun will have a red glow when the sun rises.
 * The sun will slowly shrink to its 8x8 pixel size while rising.

Clock
In the Halloween Update, Notch added a clock that lets you tell the time in the Minecraft world. It's crafted like this:

Note: The clock will not work in the Nether, due to the fact that the Nether does not receive Day or Night.

Trivia

 * In the Minecraft sky, the Sun and Moon always appear in the same position relative to the stars.
 * When looking directly at a star that is uncovered by a cloud, it will appear over the cross-hair.

Time Measurements
The approximation of Minecraft Time to real time


 * 1 Minecraft Day = 10 min (day) + 1.5 min (sunset) + 7 min (night) + 1.5 min (dawn) = 20 minutes of gameplay.


 * 1 Minecraft Hour = 50 seconds of gameplay
 * 1 Minecraft Week ≈ 2.333 hours of gameplay.
 * 1 Minecraft Month ≈ 10 hours of gameplay.
 * 1 Minecraft Year ≈ 5 days of gameplay.

The Approximation Of Real Time to Minecraft Time


 * 1 hour of gameplay = 3 Minecraft Days.
 * 1 day of gameplay = 72 Minecraft Days.
 * 1 week of gameplay ≈ 1.4 Minecraft Years, ≈ 16 Minecraft Months, = 72 Minecraft Weeks, ≈ 500 Minecraft Days
 * 1 month of gameplay = 6 Minecraft Years, = 72 Minecraft Months, ≈ 310 Minecraft Weeks, ≈ 2200 Minecraft Days
 * 1 year of gameplay = 72 Minecraft Years, ≈ 860 Minecraft Months, ≈ 3800 Minecraft Weeks, ≈ 26000 Minecraft Days

(for these calculations: 1 year = 12 month = 365 days, 1 week = 7 days, each month has same number of days, approximations: 2 digits)

of gameplay is the time you spent playing minecraft.