According to general relativity time is relative. Thus, both old earth theory and the 7 day account in Genesis could be literally true depending on what perspective one is viewing the event from.
Time exists so everything doesn’t happen at once.
It also makes more sense when one considers that It took 7 days according to Genesis, and on the 8th DAY GOD RESTED.
I do not remember ever reading about a 9th Day.
Ergo, we are living in the 8th Day.
How long is that 'day'?
It hasn't ended yet, so we can't tell. If the 8th day ever ends, then we will know exactly how long the 'heaven and Earth and the Universe' have existed.
That’s a very good point and one worth noting. Time is absolutely relative and yet we use it as an absolute in judging the age of the universe. Time is dependent upon the framework of the observer.