fun fact - ATProto requires you to store all the datetime strings in a format that's compliant with both RFC 3339 and ISO 8601 plus you cannot specify time without date.

when I worked on datetime portion of Katproto, I had this link pinned for quite some time as a reference! ah, nostalgia...

also, post factum, I find this technical decision actually good - instead of supporting all possible variations of ISO 8601 or all the variations of RFC 3339, why not just support the intersection? that way, the format will be parseable by anything compliant with any standard of these two, and you only need to handle a very small set of branches.