>>> Christoph Päper
>>> EDTF may have been developed for bibliographic purposes where the astronomical, Northern-hemispheric definition predominates, but ISO 8601 has a much broader scope.
You're absolutely correct Christoph -- EDTF obviously shares that "broader [non-bibliographic] scope" with ISO 8601. Otherwise, years would be limited to 4 digits ... that is, unless we think people will be cataloging periodicals that will be published in Y17E7 (170000000) ... or books that were published way back in Y-17E7 (-170000000). Both of the preceding example dates were taken from "4.5.2 Level 2" of ISO 8601-2. Those sorts of extremely distant dates are clearly meant for historians, curators & scientists and not to accompany ISBNs & ISSNs.