The Bible does not say that time is relative in a general sense. In 2 Peter 3:8, in speaking of Jesus’ second coming, Peter is illustrating that what we consider “slow” is not slow to the Lord. The Lord is not bound by time; He is outside of time. BUT, He created time and put us in it—in the beginning.
Genesis is a literal account of the creation, and when the Lord says it was evening and morning the first day, etc., He means A DAY, a literal day, as we understand a day. That’s not allegory.
God is an infinite Being, Who is unimaginably powerful. A Being Who IS ABLE to create the world, the stars, the universe and all that is in them—Yes, He’s more than able to do it in six days.
Of course, it's possible that, Matrix-like, we live in a universe created by God to be a perfect simulacrum of a universe in which he does not exist......