[Legate of Amon Lanc]

I don't know the answer, if there is any, but let me just tell my personal opinion. I got the feeling that at least from the point when you sort of leave the closest shores of Middle-Earth, time is not really the point - not by the Fourth Age anyway. Eärendil had his share of adventures and daily problems (at least on his original journeys, I can imagine him counting whether he still has enough supplies and so on), the length of Tar-Pharazon's journey is probably accounted for. But if I look at the description of Frodo's last journey... I honestly cannot imagine it being like "Hey, Gandalf, when are we going to be there?" "Fifteen hours. Fasten your seatbelts, please, and remain seated until further notice. At half past twelve, you will get lunch." The journey away from Middle-Earth is sort of beyond that. And it's not even, I believe, that you could determine exactly some "point of departure", like exactly pinpoint a location or time ("eight hours from leaving Mithlond") when things start to change. I would say that if you had some outside observer, like another ship following the ship which was leaving and then turning back, I guess it won't do either. I can almost see it physically in front of my eyes, if it came to recounting such event in the book, Tolkien writing something like: "...and suddenly wind came from the West, and the other ship was lost to their sight. And of those who told about the events later, some said that it occured at half past four, some said that it was hardly past three; and one sailor said that he had witnessed the lamp on the highest mast still that very evening, as if from far away, but no one else could verify his words..."

All in all, the thing is, at least in Fourth Age, as long as you are basically in Middle-Earth, you can measure time all you like. But once you are out, it does not really matter how long did it take - because you are not coming back, and also, your own perception of the journey will probably be such that you cannot put it into days or such.