Quite a while back I came to the conclusion that Faramir doesn't actually see Boromir's funeral boat, as he is convinced he does, but a vision of it, as Frodo insists. It is of course impossible to prove either way; and that is probably as it should be. The mythic aspect of Faramir's vision is far more significant than whether it is factually true. I daresay even Faramir would have thought so, regardless of what he believed. His openness to the idea that the boat could have survived the Falls of Rauros because it came from Lothlórien is sufficient evidence of this notion.
Yet the other day I noticed a detail in The Chronology of The Lord of the Rings, edited by William Cloud Hicklin, and just published as a supplement to volume XIX of Tolkien Studies. In the entry under 28 February 3019, Tolkien wrote 'First shard of horn of Boromir found' (56), and under 30 February* 'Second shard of the horn of Boromir found' (58). Hicklin comments in a single footnote to both entries (57): 'The entries regarding Boromir's horn are in pencil'. Since the Chronology is otherwise written in ink of different colors, the pencil insertions would seem to be later additions.
Now we already know from TT 4.v.667 that the two shards were found on two different days in two different places, and we know from The Tale of Years in Appendix B (1092) that Faramir saw the boat on 29* February. Thus the first shard was found on 28 February; Faramir saw the boat on 29 February; and the second shard was found 30 February. At some point before 7 March, when Faramir speaks of the shards to Frodo, word of their discovery reaches both Faramir and Denethor.
What I find curious in all this is that only The Tale of Years gives us a date for Faramir's sight of the boat, and only the Chronology gives us dates for the shards. The Chronology says nothing of the boat after 'Boromir's funeral boat sent down over Rauros' in the entry for 26 February (54). It could be that each text is telling us something different here.
The silence of the Chronology on Faramir's sighting of the boat may not prove that what he saw was a vision, but it is consistent with that interpretation. As Faramir himself tells Frodo: 'Tidings of death have many wings. Night oft brings news to near kindred, ’tis said' (TT 4.v.665).
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* In the Shire Reckoning all months had 30 days, February included.