Darkening of Valinor

The Annals of Aman date the Darkening as happening in YT 1495 (that is, the 1,495 Valian Year since the Trees first bloomed - 'YT' stands for 'Year of the Trees'). The first rising of the Moon, marking the beginning of the First Age, is dated on the same system as YT 1500. One Valian Year is equal to about 9.6 solar years, so the period in question was forty-eight years in our terms, though this must be something of an approximation. (The Annals of Aman are to be found in volume 10 of The History of Middle-earth, Morgoth's Ring).