ISO8601 is a standard managed by ISO, you can buy a digital or printed copy of the standard from here: https://www.iso.org/standard/70907.html
RFC 3339 is managed by the IETF, and their documents are accessible free of charge.
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.