Lunar years or solar years?
In the Timaeus and Critias, Plato writes that the war with Atlantis took place 9000 years before the time of Solon (who visited Egypt around the early VI century BC), giving the famous date of roughly 9600 BC.
Of course this date is problematic for some people who say it doesn't fit the official timeline of history, and thus a common claim among some Atlantis researchers is that these should be divided by 12 because supposedly the Egyptians meant Lunar years. So 9000 years before Solon divided by 12 becomes only 750 years before Solon, and if Solon lived around the year 600 BC then this would put Atlantis roughly in 1350 BC. This is an interesting date because it's close to the Sea People's invasions (circa 1276-1178 BC), a period of turmoil and changes in the Mediterranean, the collapse of the Late Bronze Age, which some believe could have inspired the story of Atlantis.
It would also make the story more historically plausible, since Athens and Sais are mentioned, along with the Greek and Egyptian civilizations, which didn't exist in 9600 BC according to official history.
However there are some issues with this theory, first of all while true that the Egyptians used a Lunar calendar early in their history, by Solon's time the Egyptians had been using a 365-day civil Solar calendar for well over a millennium (since the Old Kingdom). Egyptian scribes certainly knew the difference between months and years. So if an Egyptian priest told Solon "9000 years," the natural assumption is that he meant years, not months.
No ancient source says that these were Lunar years, Plato simply says "years" (ἔτη, etē), and all the other Greek authors who visited Egypt agreed that they spoke of thousands of years of mythic past. The Egyptians themselves used years as years in historical and administrative records.
It is ad hoc, people usually propose dividing by 12 only because they dislike the date.
This is the same thing that was done by Jews and Christians when they interpreted the Egyptian kings lists, and later the Sumerian and other kings lists that mention extremely long reigns of thousands of years. In Eusebius' comments on Manetho's Aegyptica we find the same argument, that the thousands of years of reign of Egyptian gods should be divided by 12, so 26000+ years become just 2000 years, and he says this supposedly matches the Biblical timeline from Adam to the flood, and then he also says that the gods of the list didn't rule in perfect succession but at overlapping times in different places, despite what the Egyptian sources really say...
So Eusebius and early Christians accepted the Biblical narrative as fact and tried to reconcile other mythologies with it, but little did they know that today the Bible would no longer be considered infallible by authorities, people naturally don't believe that patriarchs could have lived hundreds of years (969 years of life of Methuselah etc.), so again they say we should apply the division by 12, but if we do it we end up with absurdities: Adam fathers Seth at 130 years, dividing by 12 gives about 10.8 years old.
Seth fathers Enosh at 105 years → 8.75 years old.
Enosh fathers Kenan at 90 years → 7.5 years old.
So at least these were meant as Solar years by the authors of Genesis.
But today we also know that the Biblical antediluvian patriarchs correspond to the Sumerian antediluvian kings, the Sumerians also had a royal list, which exists in several versions with different durations of reigns, but they extend far longer than the Biblical or even Egyptian ones, in fact they use šars instead of years, units of 3600 years, Alulim was said to have ruled for 8 šars or 28800 years, which divided by 12 gives us 2400 years, still extraordinary and too long for traditional historians to accept. So there is no way to reconcile these numbers, the authors really meant them as Solar years.
That's not to say that these years must be historically accurate, there are still other explanations, they could be symbolic numbers (after all the Sumerian numbers are usually multiples of 60), they could be exaggerations etc...
Interestingly though, 9600 BC is also close to the beginning of the Holocene and shortly after the end of the Younger Dryas, which is why some researchers like me find the date intriguing. Whether that is coincidence or meaningful is a separate question, but it does mean that taking Plato's figure literally places Atlantis in a period of major post-glacial environmental change, which also makes the story of cataclysmic floods and earthquakes more plausible.
We also used to believe that no civilization existed in 9600 BC, no cities with walls like Atlantis, but then we found archeological traces of cities with walls and towers in the Middle East dating to even earlier periods, like Tell Qaramel, Jericho etc...
A lot of elements in Plato's story of Atlantis could be exaggerated, but I think we can no longer dismiss everything as a myth.
And if we take ancient texts literally, these long lives belonged not to normal men, but to gods and demigods, or people like the Adamites in the Bible who lived close to the gods. The gods could extend their lives, they had advanced scientific knowledge, and if you think that a life (or reign) of 28800 years is impossible consider this: right now on planet Earth there are butterflies which live 24 hours, and tortoises that live 200 years, that's a ratio of 1 to 730000, but they breathe the same air and are made of the same basic life components. There are trees that live thousands of years, and a jellyfish that can reverse the life cycle of its cells making it immortal (Turritopsis dohrnii), cells can divide indefinitely, but generic material ages with time due to the shorteting of the telomeres. However there is an enzyme active in stem cells that preserves their lengths.
A civilization who can master this process and create an "elixir of long life" could live indefinitely. In fact the gods of antiquity always had to take something in order to live longer, like the Sumerian food of life and the plant of rejuvenation that was coincidentally found underwater just like our jellyfish we mentioned earlier, or the apples of the Norse goddess Idunn and so on... Today billionaires are funding research into technologies to extend human lives, and then they will become just like the gods, we have come full circle.
I've been saying these things for years and I'm sure a lot of you know them too, I just wanted to share.