> but I find the idea that JavaScript (or any language) needs built-in constructs for this purpose to be questionable.

Either this, or every project starts with a 200kb date library import. In reality, I haven't met a single project that I don't need to deal with dates and timezone. And I almost always have a date.ts in my utils library because I don't want to import a giant date library either. But for real, developers shouldn't need to deal with this. These operations and constructs that every sites need should be build into the language itself.