I was just reading The Fellowship of the Ring, chapter 'The Ring Goes South' when I noticed this:
"...But your road and our road lie together for many hundreds of miles." - Aragorn
I don't know how may times this may have come up, it is possible I may have breezed through it several time while reading the book, but this was the first time I noticed it.
Now, I know Middle-earth uses the same calendar as normal life. Months are described as October, November etc. Even the seasons have the same names. But I know the year convention is not as the events till now take place in the year 3018 (I found this in the Appendix to The Return of the King, as I have a volume containing all three books.)
I don't know how the year convention works, but guessing by the way people speak and the way they interact and the whole setting of the story itself, I am guessing the events in this book took quite a long time before imperial units were even invented. I mean, this is a time when elves existed and there was a huge war against an evil cosmic creature, so even in-universe this must be pretty ancient.
What unit system does Middle-earth use? Was the miles reference just something Tolkien left in for readers to apprehend, or was it actually the imperial system that Middle-earth uses? I don't think the former is much plausible, because Tolkien was the person who created an entire universe with spectacular detail, right form languages to calendars. So, what unit system do people use in Middle-earth?