What alters the length of a day?

Researchers have found that periodic jumps generated in the earth’s core change the length of a day every 5.9 years on our planet. They found that variations in the length of the day over periods of between one and ten years are caused by processes in the Earth’s core. The Earth rotates once per day, but the length of this day varies.
A year 300 million years ago would have lasted for about 250 days and a day would have lasted for about 21 hours. Hence, the Earth’s six-year twitch alters the length of the day.