Usually, the Earth's rotation is actually slowing down so that the length of the day increases by about 1.8 milliseconds per century, on average. This means that 600 million years ago a day lasted only 21 hours. A change in speed has been happening here on Earth, but it is far too slow to notice. Millions of years ago, one Earth day was about 22 hours, and Earth's speed has been dropping for more than a billion years, with days increasing by around 2 milliseconds every century. The formation of days and nights happens due to the rotation of the Earth on its axis. If the Earth starts rotating faster, it would complete one complete spin in less than 24 hours. Hence the length of the day will decrease.