Noah was six hundred years old when the flood of waters came upon the earth. And Noah and his sons and his wife and his sons' wives with him went into the ark to escape the waters of the flood. Of clean animals, and of animals that are not clean, and of birds, and of everything that creeps on the ground, two and two, male and female, went into the ark with Noah, as God had commanded Noah. And after seven days the waters of the flood came upon the earth. – Genesis 7:6-10
This passage contains an apparent contradiction. This occurs because of the book Genesis is not written in a strict chronological order but instead in a chronological grouping of topics and ideas. In this passage, a false reading leads to the assumption that Noah and his family spent seven days in the ark before the flood.
Instead the conclusion statement of “seven days” is to complete this passage in fulfillment of God’s promise (Gen 7:4). The entering into the ark to “escape” happens on the very day the rains fall (Gen 7:12-13) and not a week in advance.