That's why I use the French term "fin-de-siècle" which is well understood by educated English speakers and yet always refers to the end of the 19th century without additional qualifications.

Define “educated” and cite the survey showing how well understood it is among “English speakers” (British English? Commonwealth Nations Generally? N.A. English Speakers? Non-native/ESL?)

I use “fortnightly” to mean “in 2 weeks” because bi-weekly is ambiguous, and while the game is hugely popular I still assume at least 1 person on any email chain with me reads that and is thinking “the fuck does he talk like a Victorian English Dandy for?”

> the French term "fin-de-siècle" which is well understood by educated English speakers

This may not be as well understood as you believe. I am an educated English speaker, who has many educated English speaking friends and family, I have never heard this phrase.

I agree with the other commenters that this is obscure. I coincidentally heard the term for the first time about 3 days ago (in the context of "socialism by fin-de-siècle" - the expectation that communism was inevitable among the left wing at that time.

Turn of the millennium?