Brethil

This dating comes from the Grey Annals in volume XI of The History of Middle-earth. The story there varies considerably from the final form (it describes Haleth as a man, and makes him responsible for leading his people from beyond the Blue Mountains directly into Brethil). The form of the tale in The Silmarillion is much more elaborate and extends over several generations, but the year I 420 seems to remain plausible as the date of the settlement of Brethil.

The word brethil appears in various contexts where it seems to mean 'birch' (for instance in the name Nimbrethil for the birchwoods of Arvernien). By contrast, in The Etymologies (in volume V of The History of Middle-earth, brethil is said to mean 'beech'. Though Brethil's own trees are never explicitly identified, its immediate neighbour the Forest of Neldoreth was known to be populated mainly by beeches, rather than birches.

It's difficult to reconcile these lines of evidence. Perhaps Tolkien simply changed his mind about the meaning of the word, but left an earlier usage in place. Beeches and birches are related types of tree, and it's possible that the Elves used the word brethil for trees of either kind.