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 practice, there are several different types of "days" that are defined astronomically. The solar day is the time for the Earth to complete a rotation including the extra "rotation" provided by advancing in its orbit. This is the "usual" calendrical day, equal to 24 hours. A sidereal day is the length of time which passes between a given "fixed" star in the sky crossing a given projected meridian. The sidereal day is 23 hours, 56 minutes, 4.1 seconds (or 86164.10 seconds), and is significant for locating objects in the sky. The difference between the sidereal and solar days amounts to 1/365.2425th of a day per day, since even if the Earth did not spin on its axis at all, the Sun would appear to make one rotation around the Earth as the Earth completed a single orbit (which takes one year).

giving days = 23.93446965 hours = 23 h 56 m 04.1 s.
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.