[Galin]

The general idea (one of them anyway) in the text Aman is that 144 years of change and growth in Aman will be equal to 1 year of change and growth in Middle-earth -- in other words, if an Elf in Aman plants a given kind of tree, in 144 years worth of time there it would change or grow only as much as that kind of tree would grow in one year in Middle-earth -- thus the world around the Eldar in Aman will not seem as fleeting as it is to them in Middle-earth, even though time remains constant. I note Legolas' commentary after leaving Lorien (to paraphrase): time does not tarry but change and growth is not the same in all places.


In any case, as Elbenprincess notes, did Tolkien really want readers to substitute 144 instead of 10-ish, keeping the old dates?


If he did it arguably brings up some issues, like how long it took the Noldor to leave Aman and reach Middle-earth for example: using the dates in Annals of Aman (which we know were written with 9.582 in mind), if plugging in the much higher number suddenly the Noldor take a huge amount of time to reach Middle-earth. Tolkien certainly was musing about having the Valian Year be equal to the Elvish Long Year in The Lord of the Rings, and he does write...

'In recording the events in Aman, therefore, we may as did the Eldar themselves use the Valian Unit*, though we must not forget that within any such 'year' the Eldar enjoyed an immense series of delights and achievements which even the most gifted of Men could not accomplish in twelve times twelve mortal years.'

... and Christopher Tolkien notes (note 4): 'We may... use the Valian unit': in other words, presumably, the old structure of dates in the chronicle of Aman may be retained, although the meaning of those dates in terms of Middle-earth will be radically different. See note 5.' Christopher Tolkien, Myths Transformed, Aman, Morgoth's Ring


But 'presumably' still looms rather large here I think; and even if JRRT really meant this, one still wonders if he himself had really looked at the full impact of simply switching numbers.