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.
The age of the Earth now is estimated to be between 5 billion and 4.7 billion years and estimates of the duration of the geologic time scale divisions have been made. Originally, geologic time scale divisions were based on the natural breaks in the stratigraphic column.