That timing is largely based upon the "begats" — which is completely misunderstood. Fundamentalists think the begats are an unbroken generation by generation account. However, in ancient times, one could say they were a son of a far distant ancestor, much the same way we think of ourselves as sons of Adam or Daughters of Eve. It is highly unlikely genealogical records were kept in an unbroken line of descent, especially for a nomadic peoples as the Hebrews were until about 950 or 930 BCE.

Additionally, the BIble itself says we cannot fathom a day in the life of God. So that seven days is incalculable.