An alternative First Age timeline inspired by Huinesoron

[James the Just]

The Second Age is somewhat less than 3,456 years (432 × 8).
The Third Age is somewhat less than 3,024 years (432 × 7).

Originally Posted by JRR Tolkien - Letter 211
*I imagine the gap to be about 6000 years: that is we are now at the end of the Fifth Age, if the Ages were of about the same length as S.A. and T.A. But they have, I think, quickened; and I imagine we are actually at the end of the Sixth Age, or in the Seventh.

So the Fourth, Fifth, and Sixth Ages should be somewhat less than 6,480 years (432 × 15 or 8 + 7).
8 + 7 = 6 + 5 + 4.

Death of Trees is 24 VY (= 3,456 [YS]) after Awaking = VY 888. First Age should then occupy 4,032 years = 28 VY. That is 3,456 (Death of Trees) + 576 sun-years (= 4 VY). But actually war lasts until 600? So FA = 4,056 [YS] = 28[VY +]/24[YS].

If the number of years of the Years of Beleriand were only 590 then it would be 4,031 solar years from the start of that to the beginning of the Third Age.
That's one year less than 28 VY (144 × 28 = 4,032).

What is interesting is that a Great Year, or a full precession of the equinoxes, was traditionally thought to be 360 Great Days of 72 years each. This would come out to 25,920 years, or 180 VY. A Great Season, as it were, would be 6,480 years (432 × 15).
But, in actuality, a full precession cycle is closer to 25,800 years. That would make a season 6,450 years (430 × 15). Maybe that could explain why the ages are somewhat shorter.