Obviously Emet, Elidibus, Lahabreha Venat/Hydelean, the Watcher, and the loloprits would know. The other Ascians would be reminded any time they got re-uplifted to their seat. Hythlodaeus and those who were the fuel for Zodiark seemed to have been in stasis and quickly did the math once Fandaniel broke the bindings. Do the moogles sort of know since their mythos is they fled the heavens when during what gets labeled as the age of the gods came to an end due to them fighting each other?

Are there surviving records from the ages pre Allag? Or do those who rummage around Allagan ruins only know that the world is at least five thousand years post sundering? Is there an old musty book/scroll/tablet that gives something like a record stating a year of some sort like that one node in Ayzs La that said no one still had attended to the escaped great wyrm in five thousand years sitting around in either Old Sharlyean properly or Gabul that we haven't found yet?

Maybe we'll get a date from the 24man since we are supposed to be delving into the myths of the 12 and they seem to have been around forever even if the name they go by is an Eorzean thing.