You may think a day is 24 hours right ? Well it hasn't always been so, the Earth's rotation is in fact slowing making days longer.
In the Earth's distant past, during the Cambrian Period about five hundred million years ago (when life was just about getting a hold in the seas and the land was barren) a day was only 20.6 hours long. This lengthening is because the Earth's rotation is gradually slowing at about 0.002 seconds a century, due to a transfer of momentum to the orbiting Moon, which is receding at about 4cm a year.
In the future, the Earth's rotation will keep slowing and days getting longer (all be it at a decreasing rate) until the moon's effect is weakened to such an extent that its hardly a factor anymore (its orbit will be too far out)
N.B.This is one of the reasons Chronologists sometimes add leap seconds to our time, to keep us in phase with the solar day that we experience all the time.
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