What the texts say. In the Hebrew text Adam is 130 when Seth is born; in the Septuagint he is 230 (Genesis 5:3). The same pattern runs through Seth (105 or 205), Enosh (90 or 190), Kenan (70 or 170), Mahalalel (65 or 165) and Enoch (65 or 165). In most cases the years after the son’s birth are 100 fewer in the Greek, so the total lifespans agree. After the Flood the pattern repeats: Arphaxad, Shelah, Eber, Peleg, Reu and Serug are each 100 years older at fatherhood in the Greek, and Nahor, in Brenton’s text, 150 years older (Greek copies differ on his number). The Greek also inserts Cainan, son of Arphaxad, who fathers Shelah at 130 (Genesis 11:12–13). St Luke’s genealogy of Christ includes this Cainan (Luke 3:36). These differences are plain in the texts (Very Strong).
The totals. Adding the Hebrew numbers gives 1656 years from Adam to the Flood and 292 more to the birth of Abram. Adding the numbers in Brenton’s Septuagint gives 2242 and 1172. Greek manuscripts do not all agree. In Brenton’s text Methuselah fathers Lamech at 167, which by the arithmetic would have him outlive the Flood by fourteen years. St Augustine already discussed this puzzle (City of God 15.11). Other Greek witnesses read 187, as the Hebrew does.
Which numbers are older? No one knows for certain (Debated). Some scholars think the Greek system revised an earlier Hebrew one; others, including Emanuel Tov, warn against simply assuming the Masoretic text is primary. The Samaritan Pentateuch has a third set of numbers. The regular hundred-year pattern suggests deliberate adjustment somewhere in the transmission. St Augustine thought the difference came from an early copyist of the Greek, and in such cases of plain fact he preferred the language from which the translation was made (City of God 15.13).
What the Orthodox Church reads. The Orthodox Church reads the Septuagint numbers. The Byzantine world dated its years “from the creation of the world” on the Septuagint chronology, with year one beginning in 5509 BC. The Ecumenical Patriarchate used this era until 1728, and Russia until 1700. The Biblical Family Tree on this site shows both sets of dates, Septuagint first.
How translations handle it. Translations from the Hebrew, Protestant and Catholic, follow the Masoretic numbers and have no second Cainan in Genesis 11; their Luke 3:36 still names him. Brenton’s translation follows the Greek.
What it does and does not change. No Creed or Council fixes the age of the world. The genealogies teach that God’s promise ran through real generations from Adam to Abraham and on to Christ. The two sets of numbers change how long that history was, not who God is or what he promised. Christians on either side should hold exact dates loosely.