@shoover "football" means two different ball games in UK and US.

@shoover Even one of our mods, @cag51 (who lives in US, I think), thought it's "fortnights", not "fortnight" in their comment below the question. So, you can imagine how uncommon this word is in US. I lived in US for over 30 years, I never heard of "fortnight", two weeks or bi-weekly are very common words. I used to have bi-weekly pay checks, people always say two weeks pay. No one ever said fortnight pay to me.

@Nobody Yes, that's why I said "unlike football..."

@Nobody I've lived in the US (where we say "in the US," not "in US") for all of my 58 years and I've known the word "fortnight" since childhood. Maybe it's a generational difference...

@shoover Let me emphasize my point in my answer. When you give a set of problems in an exam for your students, you want the problems to be as clear as possible to avoid possible confusions, right? After all, you want to test the students' chemistry, not their English, right? Of course, they should be able to read and write English. But, this question is about dialects, not the English language. "14 nights" or "14 days" are more clear and precise, no way to be confused.

@shoover If the students cannot parse "14 nights", how did they pass other classes? But, as a non-native English speaker living in the US, I did not know the word "fortnight". For others, it may sound weird. For me and some others, it's normal. In these days, we are talking about "inclusive" all the time, do you think the word "fortnight" is inclusive given the comments above and your personal experience?

@shoover Please read this answer and others if you're interested.

In particular, in the comment section, "Yeah, don't use 'fortnight'. Unless you intentionally want to be not understood or sound like you woke up from a 200 year coma."

And the user who made this comment has >70K rep on English.SE.

I'm a native metric and loved furlong–firkin–fortnight.

@Nobody I'm a native US-English speaker, and I mostly agree. As others have said, I'd expect most US college students to know the meaning of the word (though I'm 52, so times might have changed), but if someone actually uses the word outside of very few contexts, it's going to strike me as being a tad affected. Of course, humor is one of those valid contexts, as perhaps you're trying to evoke affectedness.

@Nobody While fortnight may be antiquated, and non-native speakers may not be familiar with the term, it's extremely rude for you to stomp your feet, huff an puff, and pronounce that Americans don't know what a fortnight is. it's often repeated through out public education through american history courses, as certain quotes and passages require it's understanding, so it would be a tad bit unusual to find even younger adults who don't know what a fortnight is.

@shoover @Krupip anecdotally (as an American) while I am familiar with the term through exposure in literature, I have never heard or used "fortnight" in conversation. It seems more rude to me that you would expect the average American to know this word - it's not a matter of ignorance, but of usage: I cannot think of a situation in which "fortnight" would be a better fit than "two weeks"

@Krupip to be fair, young adults DO know what fortnight is xD en.wikipedia.org/wiki/Fortnite

@shoover 'I've known the word "fortnight" since childhood' Have you ever used the word "fortnight" instead of "two weeks", and in what contexts? I grew up being exposed to classic literature, but I can't imagine why someone who has not would necessarily know the word.