I decided to pull all the competing timelines into a table:
And by "all" I mean "both.
Letter 211 aligns with the astronomy argument given by
James the Just; perfectly so if you accept their point that "about 6000 years ago" should be read as "the length of the 2nd and 3rd Ages combined" (ie, about 6500 years). Meanwhile, the NoME timeline aligns with the date of the last thing that can reasonably be called an Ice Age (the Younger Dryas). I've tweaked the 4th-6th ages to be "gradually quickening" (the original NoME timeline had 3 2433-year ages), which makes it align pretty closely with the PoME "100 generations".
In terms of real history, the Astronomy Timeline has the 4th Age end with the collapse of the Old Kingdom of Egypt, and the 5th with the birth of Christ. The NoME Timeline shifts those markers to the ends of the 5th and 6th Ages. All other Ages are solidly prehistoric.
It's pretty funny that we've ended up with a Long and Short timeline, one supported by astronomical evidence, the other by geological (the Ice Age). Actual archaeology of the ancient Near East
has exactly the same problem, with the Sack of Babylon wandering by over 200 years depending on how you count it. Makes it feel more real, somehow.
EDIT: Given that it's Tolkien, it's also interesting to note that Bible chronology suffers from the same problem: dates for the Genesis creation have historically clustered around either 4000 BC or 5500 BC. Neither of these dates really fit anything other than the simple Letter 211 chronology - a literal 6000 years since Gollum tripped into a volcano. (This means that the Genesis flood is
not connected with the fall of Numenor; it falls sometime in the 4th (Astronomy timeline) or 5th (NoME timeline) Age.)
hS