Right, but the JS and JVM runtimes have quite different behavior. And I'm willing to bet that implementing something as non-trivial as java.time again in Scala(.js) will end up causing quite a few bugs because they aren't identical.

That's why I'd rather have a single implementation for anything non-trivial and just retarget that to each backend using more low-level primitives. After all you're using a different language -- using a different date/time API shouldn't be much of a problem.