Complex sentence: During the last 620 million years the period of rotation of the earth (length of a day) has increased from 21.9 hours to 24 hours; in this period the Earth has lost 17% of its rotational energy.
Split into simpler sentences: During the last 620 million years the period of rotation of the earth (length of a day) has increased from 21.9 hours to 24 hours. In this period the Earth has lost 17% of its rotational energy.