Days and Time began on DAY ONE of the Creation WEEK when the LIGHT was created. The orbs, (sun, moon and stars), were created on day four to regulate the LIGHT that was created on DAY ONE; the sun giving Bright Light by DAY and the moon and stars giving Medium to Low Light by NIGHT. The function of the orbs are to work like dimmer switches to regulate the Light Energy that was created on DAY ONE:
Genesis 1: 3-5 And God said: 'Let there be light.' And there was light. 4 And God saw the light, that it was good; and God divided the light from the darkness. 5 And God called the Light Day, and the darkness He called Night. And THERE WAS EVENING AND THERE WAS MORNING, DAY ONE.