The Archbishop James Ussher is responsible for the calculation, widely believed among evangelical christians and other wack-jobs, that the earth is only 6000 years old.

He started with the beginning of Saul’s reign as the first king of Israel. There was consensus that this had happened around 1020 BCE. He then worked backwords through the Bible, adding up all the years until he got to Genesis.

Even if you believe in the literal truth of the Bible, Ussher's math was nonsense. For example, he figured that the rule of Judges lasted 330 years. However, modern theologians believe that each Judge controlled separate tribe(s), so that their interval of rule overlapped, and the intervals specified in the Bible should not be simply added up.

For those who want to celebrate an anniversary party, Ussher (and others who don't believe in geology) say that the earth was created on October 22, 4004 BCE.