How were there 'days' before the creation? How did the writer of the Genesis determine the duration of 'days'?

According to ordinary of thinking 'a 'day' is considered by one spinning of the earth on its axis that makes the sun to appear and disappear once for each. If God did certain things on the First Day how did He consider that day as a 'day' because there could have been no sun or earth in the beginning of beginnings. What is the duration of the 'days' in which God rested for one and the six days before that.?