I'm pretty sure that fortnight used to be more commonly used in the US, but it has fallen out of use in the past century or so. So to an American, probably the best comparison is the word "score" to mean "20." That was also in common use, particularly to describe years or age, a century ago. But except for the famous Gettysburg Address that begins "Fourscore and seven years ago," most Americans would never have any reason to know what it means.
I thought of the same example (as another fellow American).
My guess is that most Americans know that Abe Lincoln said, "Four score and seven years ago..."
But my guess is that a very small percentage of those Americans know what it actually means: "87 years ago."
It's one of those terms that's fallen out of common usage. So even if somebody remembers a quote with that terms, they may not know what it means or why.
ETA: One thing I've always wondered (that I guess there is no way to tell, given the lack of video or tape recording back in the day), but did people really talk like characters in old-fashioned literature? By which I mean 17th, 18th, early 19th century. Or were those books written for a more highbrow crowd, so the authors tried to "write fancy"? For example, 87 years ago is so much simpler to say than "four score and seven." Although it was a political speech, and even today, people don't normally talk like politicians do. But I'd be curious to learn how people actually did speak.