For simplicity one Earth Day is assumed to be 24 hours long. However the exact time taken for the Earth to rotate once on it's axis is 23 hours, 56 minutes, 4 seconds and is known by astronomers as a sidereal day.
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There are two periods that can be defined as a day on Earth.
One is the sidereal day, the time it takes the Earth to make one rotation as observed from a fixed position away from it. This rotation takes 23 hours and 56 minutes and 4 seconds.
The other is the observed or solar day, which has been established as exactly 24 hours. The difference is due to the fact that the solar day is measured from noon to noon, and the Earth is orbiting the Sun in a counter-clockwise direction as it spins (also counterclockwise, west to east). It takes that additional 4 minutes to reach the same relative position to the Sun.
Other than the adjustments called leap seconds, the solar day has been established as exactly 24 hours or 86400 seconds.
*Due to Daylight Savings Time and its varied usage, there is one annual calendar day which will only last 23 hours (spring ahead) and one that lasts 25 hours (fall back)