> On August 17, 2017 at 8:54 AM Christoph Päper <[log in to unmask]> wrote:
> but it is not incompatible with any format, so it would be a valid extension if the partners in information exchange agreed upon it.
The ISO 8601 standard has a number of cases which are explicitly mentioned may be done, but only upon agreement of the data exchange partners. I've always interpreted this as meaning that extensions that are not mentioned at all are not acceptable, even if the extension doesn't contradict any part of the standard. Certainly parties who made such an extension would be unprotected against future updates to the standard that turn out to be incompatible with the parties' extension.