Brian Palmer at Slate investigates how creationists calculate the Earth’s age. On face value, the bible provides some chronology:
The Bible provides plenty of internal chronological information. Adam lived 930 years, and his son Seth 912 years. The Israelites lived in Egypt for 430 years “to the very day.” Saul reigned as king of Israel for 42 years. Summing up the dates is tedious, but it’s doable.
The real trick is filling in the gaps between the events:
Irishman James Ussher, the 17th-century archbishop of Armagh, famously solved this puzzle by comparing events in the Bible with histories from other civilizations. Most critically, Ussher found a reference to the death of Babylonian king Nebuchadnezzar in theSecond Book of Kings. Ussher then used Ptolemy’s history of Babylonian kings, combined with Greek historical events of known dates, to pinpoint the death of Nebuchadnezzar in 562 B.C. Adding together the generations of Old Testament begetting and the reigns of kings, Usher surmised that 3,442 years passed between the creation and Nebuchadnezzar’s death. Ussher thereby arrived at his now famous estimate for the Earth’s creation: 4,004 B.C. He eventually went one step further, marking the Earth’s birthday as 6 p.m. on Saturday, Oct. 22, 4004 B.C.
There are, however, discrepancies in how creationists calculate these events:
Most of these variations result from differences in Old Testament interpretation. For example, one of Ussher’s greatest dilemmas was choosing which text to follow. The Greek Septuagint version suggested that 2,242 years elapsed between the dawn of time and the biblical flood. Ussher rejected that estimate because, if it were accurate, Armageddon should already have occurred. (Seventeenth-century theologians thought the earth would end after 6,000 years.) The Samaritan Pentateuch suggested 1,307 years between the creation and the flood, but Ussher eventually went with the traditional Hebrew text’s 1,656-year-estimate. Harold Camping’s methodology in arriving at a vastly different date is perplexing. He added together the lifespans of Old Testament fathers and sons, assuming that their lives didn’t overlap.