ISO 8601 supports all sorts of corner cases, apparently. But it costs money to get the full standard, unlike RFCs. Someone bought a copy and was answering questions few years ago:
https://www.reddit.com/r/ISO8601/comments/mikuj1/i_bought_is...
https://www.reddit.com/r/ISO8601/comments/mikuj1/i_bought_is...
RFC 3339 is managed by the IETF, and their documents are accessible free of charge.
Or from your national member body, which for some standards and countries can be substantially cheaper.