javax.time is considered by some to be one of the best, most well-designed general and versatile libraries for date and time related matters out there, regardless in any language.

How would things be improved by throwing that away, and inventing something different? I think keeping the API and providing an implementation for Scala.js is a much more reasonable approach.

Designing a date/time requires very rare skills and knowledge that most people lack. A re-implementation of javax.time means all the documentation, tests and existing code can be re-used and allow correctness checks against the existing implementation.