Genesis 1:5

And God called the light Day, and the darkness he called Night. Time had begun for the earth, and therefore the Lord made this rule for the division of light and darkness, as they follow each other in regular order, and did so even before the creation of the light-bodies. He Himself defined the unit of time which He thus ordered. And the evening and the morning were the first day. And there was evening, when the darkness had ruled alone, and there was morning, when God's almighty power created light and separated it from the darkness. Ever since the first day of the world the regular recurrence of darkness and light marks the period of one day, as we now divide it into twenty-four hours. This is the fundamental meaning of the Hebrew word here used, which must be assumed even Psalms 90:4 (Cf 2 Peter 3:8), where the Lord accommodates Himself to human speech and limitations, for the sake of comparison.