Chris Myrick linked to the relevant Wikipedia entry to mark the 6013th “birthday” of the Earth.
[James] Ussher [Anglican Archbishop of Armagh] deduced that the first day of creation began at nightfall preceding Sunday, October 23, 4004 BC, in the proleptic Julian calendar, near the autumnal equinox. [. . .] Ussher’s proposed date of 4004 BC differed somewhat from other Biblically based estimates, such as those of Bede (3952 BC), Ussher’s near-contemporary Scaliger (3949 BC), Johannes Kepler (3992 BC) or Sir Isaac Newton (c. 4000 BC). Ussher’s specific choice of starting year may have been influenced by the then-widely-held belief that the Earth’s potential duration was 6,000 years (4,000 before the birth of Christ and 2,000 after), corresponding to the six days of Creation, on the grounds that “one day is with the Lord as a thousand years, and a thousand years as one day” (2 Peter 3:8).
