[dahauns]

Nah, that's not a "sacrifice", but the only sane way. In the ideal case, clearly document the constructor with a warning that it's not ISO conformant and offer a ISO conformant alternative.

In my (unfortunate) experience, DateTime/Timezone handling is one of the things most prone to introduce sneaky, but far-reaching bugs as it is. Introducing such a behaviour change that (usually) won't fail-fast, will often seemingly continue working as before until it doesn't and is deceptively tricky to debug/pinpoint/fix ist just asking for a fast lane into chaos.

And even with JS going the extra mile on backwards compatibility, I don't think most other languages would introduce that kind of breaking change in that way either.