The day is (roughly) defined as the time required for the Earth to complete a rotation. The length of the day is not a constant over long timescales (i.e., millions of years), but changes over the course of time as the Earth's rotation slows due to the tidal torque from the Moon . The current deceleration is radians per second squared,

In civil calendars, the length of individual calendar days may be adjusted as a result of daylight saving time (by adding or subtracting one hour out of the usual 24) or addition of a leap second .

The plot above shows the duration of daylight (as defined by the interval between sunrise and sunset ) for Chicago in the year 1999. The length varies from a minimum of 9:20 on the winter solstice of December 22, 1999 to a maximum of 15:02 on the summer solstice of June 21, 1999.

Day of the Week , Daylight Saving Time , Hour , Leap Second , Month , Sidereal Day , Solar Day , Summer Solstice , Sunrise , Sunset , Winter Solstice