If you measure the day as we measure the day now. Then the very first day was 1/ 100,000 of a second long. In that amount of time, the universe went from the size of a mustard seed, (some say smaller) to the size of our solar system. Day two was 2/ 100,000 of a second & the universe doubled in size.
It has continued to increase at that rate to the point were we are today with 24 hour days.
It has continued to increase at that rate to the point were we are today with 24 hour days.