Dinosaurs! And a bit more on time...

One second used to be defined as 1/86,400 the length of a day. However, Earth’s rotation isn’t perfectly reliable. Tidal friction from the sun and moon slows our planet and increases the length of a day by 3 milli­seconds per century. This means that in the time of the dinosaurs, the day was just 23 hours long.

http://discovermagazine.com/2009/mar/20-things-you-didn.t-know-about-time