How long for the earth to rotate on itself?

Long, long ago, people noticed that every so often it became warm enough to plant things, and later to harvest them. They noticed that after this, it got colder and colder for a long time, then warmed up again until it was once more time to plant things. They called this period of time the equivalent of "Year". So, a year is the time taken by the Earth to make one full rotation on itself.