City of God - Book 15, Sections 12 and 14
Augustine addresses objections and controversies regarding the longevity of ancient man. He writes that some well-meaning individuals, in an attempt to make the genealogies believable, have concluded that a “year” must have meant something different to the ancients, possibly a relatively short amount of time like one month or four months. He admits that there were in fact some ancient societies who counted time like that, such as the Egyptians, for whom a “year” was four months.
Of the opinion of those who do not believe that in these primitive times men lived so long as is stated.
For they are by no means to be listened to who suppose that in those times years were differently reckoned, and were so short that one of our years may be supposed to be equal to ten of theirs. So that they say, when we read or hear that some man lived 900 years, we should understand ninety, […]The younger Pliny, after mentioning that some writers reported that one man had lived 152 years, another ten more, others 200, others 300, that some had even reached 500 and 600, and a few 800 years of age, gave it as his opinion that all this must be ascribed to mistaken computation. For some, he says, make summer and winter each a year; others make each season a year, like the Arcadians, whose years, he says, were of three months. He added, too, that the Egyptians, of whose little years of four months we have spoken already, sometimes terminated their year at the wane of each moon; so that with them there are produced lifetimes of 1000 years.
Augustine does not flat-out deny the possibility that a Genesis “year” does not mean a literal year, but he says he thinks it is very unlikely because compressing the timeline in this manner would mean, among other things, that Cainan was only seven years old when his son was born.
By these plausible arguments certain persons, with no desire to weaken the credit of this sacred history, but rather to facilitate belief in it by removing the difficulty of such incredible longevity, have been themselves persuaded, […]But there is the plainest evidence to show that this is quite false. […] But what shall I say of his son Cainan[?] […] [W]hat man of seven years old begets children?
Augustine also defends the literal Genesis year by pointing out that it provides a plausible explanation for the Genesis population boom.
For there can be no doubt that, the lives of men being so long, the first-born of the first man could have built a city,—a city, however, which was earthly […].