I have often wished for that API to be available in other environments, so this is definitely a welcome addition to JS.
Note that the authors of the library are confused -- Threeten-(backport) and Joda-Time are different libraries with different behaviors, and one isn't an "aka." for another. However, Joda-Time's author went on to develop Threeten based on lessons learned, and this library purports to be a JS port of Threeten-Backport.