Days are actually getting longer due to changes in the earth's rate of rotation!

The length of a day is defined as the time it takes for the earth to rotate once on it's axis. At the moment we use 24 hours for this time period. But a day was not always 24 hours long.

There are many things that affect the rotation of the earth. In fact, any significant weight that shifts on the earth will have an effect on the rotation. This is due to a physical phenomenon known as the "moment of inertia."

Think of a figure skater. If the skater wants to twirl faster she draws her arms in close to her body, thereby reducing her moment of inertia, and increasing her rate of spin. If she wants to slow down, she stretches her arms out.

The same effect is seen with the earth. The moon's attraction pulls the water in the oceans towards itself, thereby causing the tides. But, at the same time, this tidal "bump" that forms in the ocean serves to decrease the earth's moment of inertia and hence its rate of rotation. It's a bit like the earth stretching its arms out. Thus, at every tidal cycle, a fraction of time is shaved off the earth rotation.

But, there is no need to panic. The current rate of slowing is only 0.005 seconds per year, so it is not exactly going to mess with your birthday plans for next year.

Doing some calculations, however, and we can see that 370 million years ago, around the time of the dinosaurs, the length of a day would have been only 22 hours.