Archbishop Ussher took the genealogies of Genesis, assuming they were complete, and calculated all the years to arrive at a date for the creation of the earth on Sunday, October 23, 4004 B. Of course, even assuming the method was valid, such an exact date is not possible from the genealogies of the Bible (Ussher assumed all the years the patriarchs lived were exactly 365.25 days long and that they all died the day before their next birthday).
There are a number of other assumptions implicit in the calculation.
In reality, the Bible makes no claim as to the age of the earth, although it does establish a minimum age.
This page examines some of the history of the controversy—what the Bible actually says and does not say—and the scientific evidence surrounding the age of the earth.
The first, and foremost, assumption is that the genealogies of Genesis are complete, from father to son throughout the entire course of human existence.