Besides the conflict with Sauron having died for 500 years as per start of first movie vs several thousand years ago as per Elrond in 2nd movie, I found it odd that in Fellowship of the Ring they said "on this day October the 24th...." at Rivendell
October is based on something else and completely unknown to these guys. I thought they should have had their own calendar system.
Tolkien created a calendar and had the races of Arda revise it, just like people didn't hop into spontaneous civilization with a full, 12-month calendar at our disposal.
Since part of his concept was that his world was a kind of mythology version or alternate version of Earth, it makes sense that they'd have a 365 (ish) day solar cycle and wind up with 12 months, for the same reasons we did. Although they wound up with slight differences.
Then, because a further aspect of the conceit of Tolkien's works is that he was a "translator" of history books he "found" what we are reading (and watching) are translated.
All this is to say that Elrond would have called it "Narbaleth", but he was speaking English for the common good of those assembled, and as part of the "translation" of Tolkien.
Here:
https://lotr.fandom.com/wiki/Calendar
It's not a goof. Tolkien wrote what is basically a Mythology for England.
It's not a goof. In the appendices to "LOTR", which are as long as your average Fantasy book, Tolkien made it clear that his characters were not speaking English or using our calendar, that the languages and calendars the characters were using were being translated into English.
I honestly don't remember if Tolkien went into what calendar they used in Middle-Earth, but I wouldn't have put it past him. Tolkien was, like his hardcore fans, a massive nerd.