100 million years ago, days only lasted 21 hours!

Whose fault is it? The Moon’s! The moon is relatively large as a satellite, so its effects on the earth are comparable to the effect of another planet orbiting the Earth. The moon orbits the Earth parallel to the Earth's orbit around the sun, rather than perpendicular to the Earth's path.

The Moon is big enough to cause substantial effects on Ocean tides on the Earth. There is a tidal bulge that follows slightly ahead of the Moon’s orbit around the Earth. That, combined with the fact that the Moon has a parallel orbit causes torque on the Moon, and it accelerates its orbit. In return, however, this decreases the speed of the Earth’s rotation.

The amount is very small, so much so, that in 100 million years the rotation of the earth slowed only 3 hours. For more detailed information on the way tidal acceleration works and the scientists who discovered the effect, see the source.