May be we must pull in "Myths transformed" hear?
The timeline with about 590 years for the first age is clearly based on the untransformed myth. Were the first age begins with the first raising of the sun. (this myth you can find in "The Silmarillion" and a fuller timeline is given in "The History of Middle-Earth": volume 11: "The war of Jewels")
In "Myths Transformed" ("The Histairy of Middle-Earth"; volume 10: "Morgoth's Ring") Tolkien changed his mind about the timeline and astronomy of Middle-Earth greatly.
So it might be that the first age of that transformed Myth would start with the awakening of the children of Ilúvatar. But we do not know because Tokien hadn't left a timeline for that myth.
In the Letter, that say we are at the end of the 6th or the beginning of the 7th age (I do not know the numbers out of my head) is also a footnote that makes clear that Tolkien imagined a time gap between his time and the fall of Baradur of 6000 years.
So that is the fact to start with. I have made calculation with that approximation and the time of the recorded moon phases in the Hobbit and the Lord of the Ring. It is a rather complicate calculation and is not jet oneline, so that I can't give a link to the summary. (But I wouldn't help you to much, because it is as jet only in German - may be it is time to make a translation.)
For short the outcome was quiet simple: Taking the year of the letter an the year of adventure of Bilbo (from which I started) into account the 6000 years were not a approximation but perfectly to the point. The moon phases agreed to the tale in a period of about 7 years but with less good fit for the other years around the time gap of 6000.
So if you like to know how long it since what ever event in Middle-Earth take the year of the letter (which I can't remember know), go back 6000 years and you will land your self in 3019 Third Age.
Respectfully
Findegil
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