The increase in length of day on the earth at a rate of 2.4miliseconds/100 years is due to

Tidal friction has lead to increase in daylength by 2.4 milliseconds/100 years by decreasing the Earth's rotational speed.

A large part of it is due to something called tidal braking, which basically means that the Moon's tidal pull is acting like a drag on our planet. "The heaping up of water drags on...