The rotation speed is at its maximum in late July and early August and at its minimum in April; the difference in the length of the day is about 0.0012 second. Since about 1900 Earth’s rotation has been slowing at a rate of approximately 1.7 seconds per year. In the geologic past Earth’s rotational period was much faster; days were shorter and there were more days in the year. About 350 million years ago, the year had 400 to 410 days; 280 million years ago, a year was 390 days long.