See the contributing guidelines of Scala.js core. The same guidelines apply to this repository.
scalajs-java-time contains a reimplementation of part of the JDK in Scala.js itself.
To contribute to this code, it is strictly forbidden to even look at the source code of the Oracle JDK or OpenJDK!
This is for license considerations: these JDKs are under a GPL-based license, which is not compatible with our BSD 3-clause license.
The only existing JDK source code that we can look at is the dead Apache Harmony project.