Are you familiar with Einstein's theory of special relativity. The way we see space and time, is reference dependent. Space and time is not the same everywhere. Space or distance can appear to contract and time can appear to slow down as a function of velocity. If I; ghost, was traveling as the speed of light, which mass cannot do, the universe would appear to shrink to a point, and my time; clock would appear to be stopped, relative to the earth.
In the speed of light reference, the universe would appear to come and go in an instant. For God to see the 15 billion year old universe progress in a day in his reference, he would have to be close to a speed of light reference. Plug in the numbers into the special relativity equations and solve for t.
In physics, there is a thought experiment called the twin paradox, where two twins start on the earth. One twin leaves in a rocket, approaching the speed of light. When he returns, he hardly ages, but his brother who stayed on the earth is now an old man. There are two different rates at which time propagated, with the moving twin seeing earth time pass like time lapse photography, and his brother getting older very fast. The moving twin sees his clock running very slow so he hardly ages. If God was the moving twin, the universe would seem to age; evolve, quickly, but he does not age. Many years can pass on earth, but to him only seconds have passed.
Genesis and Science can agree if we use the twin paradox and special relativity, with God on the moving reference that starts at the speed of light. This reference slows gradually from the speed, of toward the earth reference time, until God become man; Jesus, where both reference are the same.
You do no have to accept that, but it can be supported with well established science and math using a hypothetical example for academic purpose. God, by definition, can do such things. Time dilation is part of the universe, such as in the center of the primordial atom of the BB.
When the universe started to expand there is no earth day, only highly time dilated BB days. Science is wrong by using earth days; 15 billion years since it ignores initial extreme time dilation, which if included, would lower the days based on only references that did exist, instead of pretend earth years that ignore relativity.