Board Thread:General Discussion/@comment-6192846-20150304021219/@comment-6986530-20150409214708

The idea that the negative numbers resulting from using the subtraction method on BSS and Watching Us correspond to dates in-canon, as opposed to just meaning they're the odd two out among the canon songs, is only a theory.

It's a likely theory, but I don't see it as definitive confirmation of their canonicity. BSS's negative number corresponds to its release date, for example, so the numbers aren't necessarily significant as in-canon dates.