100 million years ago, a day was only 21 hours long, not 24.

The 24-hour clock is locked into our mammalian biology, our technology and our culture. But it hasn't always been that way.

The length of an Earth day has been increasing slowly throughout most of the Earth's 4.5-billion-year history, says Dr Rosemary Mardling, mathematical scientist at Monash University, and it all has to do with the Moon. 
"The reason is that the Moon is attempting to slow down the spin of the Earth. The Earth was spinning very much faster when the Moon was formed," says Mardling.
Back when the Moon was formed the length of an Earth day was a very brief two to three hours, and a much closer Moon was orbiting the Earth every five hours. 
So how did the Moon slow us down? It has to do with gravitational force and the transfer of angular momentum.
"If someone was sitting on a chair that could spin and you tried to slow them down with your hand, they would slow down a little bit and you'd be flipped around a bit. You'd get some angular momentum."
And that's what is happening with the Earth-Moon system. Much like the hand interrupting the spinning chair, the gravitational pull of the Moon exerts a force on the Earth that transfers angular momentum from the spin of the Earth into the orbit of the Moon. 
"In doing so, the Earth slows down a little bit and the Moon moves away from the Earth," says Mardling.
We can measure the speed of the Moon's retreat — reflective panels on the Moon allow for fine calibrations that show that it's currently moving away one to two centimetres a year. 
We also know that the spin of the Earth is slowing.
"The spin down rate is very slow," says Mardling, "It's about two milliseconds per century. So the Earth's day is getting longer by a 500th of a second every century"