ISO 8601 also has notations for all sort of stuff the RFC doesn’t need, like uncertain and approximate dates (for libraries or museums perhaps), as well as repe

ISO 8601 also has notations for all sort of stuff the RFC doesn’t need, like uncertain and approximate dates (for libraries or museums perhaps), as well as repeating events, durations, intervals, and such (some of that part is a riff on iCal, or so I’ve heard); most of it is in 8601-2 (which is a thing since the 2019 revision) but some parts are in -1.