The Grey Annals in volume XI of The History of Middle-earth give a date of I 420 for the settlement of Brethil. However, 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). This dating scheme does not fit easily (or indeed at all) with other accounts, where I 420 is given as the date of Haleth's death.

There is no other definitively dated account, but we do have a suggestive document in the form of a genealogy of the House of Haleth, later in the same volume of The History of Middle-earth. That chart dates Haleth's father and brother as both dying in I 375 (and indeed Haleth's father Haldad is specifically noted to have been slain in an Orc-raid). So, we can safely date Haleth's ascendancy to the leadership of her people to the year I 375. In principle, this might also have been the date of the settlement of Brethil, but in practice, the process of gathering her people and leading them across Beleriand probably took some time, and so the actual date of settlement might easily have been more than a year later, or perhaps even longer. Nonetheless, lacking any more detailed dating information, we give a date of approximately I 375 here.

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.