Harder than it looks.Some of you are probably aware of on James Ussher, Archbishop of Armagh and Primate of Ireland. Back in the seventeenth century, Ussher was a highly respected theologian and a very learned historian. His main feat was to use the Bible as the guiding light in determining the age of the Earth. Remember, at this time, the Bible was often seen as a literal history of the world rather than a figurative collection of stories representing the Word of God. Also (and just as important), there was little other data at this point. No geological records, no nothing. Ussher used what he had, and he did that quite brilliantly.
Now, some of you are probably aware of the 'begat' section of the Bible. You know, like Genesis, chapter 5, verses 9 on. And Enos lived 90 years, and begat Cainan... And Cainan lived seventy years and begat Mahalaeel... Now, you can get up through Noah and up to Abraham and Isaac using this method, but then it gets hairy.
Beyond here, you'll still find the occasional 'begat', but no years are recorded. Thirty-nine full generations are recorded in the Bible from Jacob, the son of Isaac, to Jesus, the Son of God. No dates. Ussher had no choice but to go through and start walking through each generation, step by step, day by day, and figure out how long each took.
This is understandably arduous. You would have to count seasons (for that's what you get recorded in the Bible during this period) and attempt to match up any speck of data you have to other known world events during the time, cross-referencing with the best histories left from antiquity. This involved massaging all the dates given in Babylonian and Egyptian years into the Julian calendar, comparing events, and then tracking back the equinoxes. Eventually you'll get to an autumnal equinox on a Sunday (Biblical day of the creation of the world) happening on October 23rd in the Julian calendar on 4004 BC, very close to the earliest estimated history, the Chaldean histories, which places it at 4152 BC.
This led to Ussher's famous (and now ridiculed) pronouncement : "the beginning of time... fell on the beginning of the night which preceded the 23rd day of October, in the year... 4004 B.C.".
The science author James Trefil recommends the following simplified equation - add up the begats in the Bible until Noah (when the flood hit and the lifespan of humans shrunk noticeably) and then use the average of the begats from Noah to Isaac (50 years) as an estimate for the following 38 generations. You'll end up with 4003 BC. Here's a quick run-down, so you can see for yourself :
Creation - 7 days, and then came...
Adam - 130 years, and then came...
Seth - 105 years, and then came...
Enos - 90 years, and then came...
Cainan - 70 years, and then came...
Mahalaleel - 65 years, and then came...
Jared - 162 years, and then came...
Enoch - 65 years, and then came...
Methuselah - 187 years, and then came...
Lamech - 182 years, and then came...
Noah - 500 years, and then came...
Shem - 100 years, and then came...
Arphaxad - 35 years, and then came...
Salah - 30 years, and then came...
Eber - 34 years, and then came...
Peleg - 30 years, and then came...
Reu - 32 years, and then came...
Serug - 30 years, and then came...
Nahor - 29 years, and then came...
Terah - 70 years, and then came...
Abraham - 100 years, and then came...
Isaac - 60 years, and then came Jacob, where it gets messy.
Supposedly, in geological circles in universities the world over, much alcohol is drank on October 23rd to commemorate (and/or make fun of) Ussher's famous calculation.