Sounds like we need an industry/language-wide test suite to check these many date/time/calendar libraries against. Like the browser acid tests, though focused to baseline functionality only.
https://en.wikipedia.org/wiki/Acid3
I like this new lib (Thank You) but the name unfortunately implies the opposite of what it is. "Whenever" sounds like you don't care, but you'd only be using this if you did care! Also Shakira, haha. Hmm, pedantic is taken. Timely, precise, punctual, meticulous, ahorita, pronto, etc. I like that temporal name.
Finally, none of these links mention immutability, but it should be mentioned at the top.
Without the slightest sense of irony, I actually strongly suspect such a test suite would only be valid at one moment in time, since the timezone legislation is almost continuously in flux. That's why <https://www.iana.org/time-zones> and its friend <https://www.oracle.com/java/technologies/javase/tzupdater-re...> exist. As if to illustrate my point, the latest update was 2025-03-22, presumably nuking any such conformance test from Mar 21st
In that case, you'd have unit tests that confirm behaviors like compatibility or failure of some operations between types and integrations tests which pull an up to date DB of rules and tests against that.
It would have to take the real world into account, no? Additionally it could test various timezone definition permutations without necessarily being dependent on a real one.