The principle of eternal is based upon two immutable things: it has no beginning, having always existed; neither is there any variableness nor shades of turning representing that it does not change in form or substance. So theoretically, time did not exist until the beginning the expanse of space and only exists within that expanse. Even still, the expansion of space would not subjugate the eternal One to time since it is not within the expanse of physical body of space.
The first day began with the darkness that was upon the face of the expanse of space was created in the beginning.
The first day concluded when the visible light was formed in the midst of the heaven.
The measure of a day is the evening and the morning which is first day. There is no gap, the first day begins with
Genesis 1:1 and concluded with
Genesis 1:5. The Sun, moon and celestial bodies came into existence during this period referred to as the first day.