Scientists first make observations that raise a particular question. In order to explain the observed phenomenon, they develop a number of possible explanations, or hypotheses. This is the inductive part of science, observing and constructing plausible arguments for why an event occurred.
As indicated earlier, the Bible does not fix the age of the earth, contrary to the claims of Answers in Genesis. Historically, their claim comes from the work of James Ussher, Bishop in the Church of Ireland, from 1625 to 1656.
It goes like this: If the age of the universe is truly about 13.7 billion years old as scientists have ascertained and if God truly created the universe in 6 days, as inspirers of the Bible have ascertained, then each Biblical day should be 13.7 billion years (13.7 x 10e9) / 6 = 2.283 billion years long.