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