Draft versions of the ISO standard are easy enough to find on the internet. But the Wikipedia article is excellent and covers just about everything.
Yes, and I've read the drafts of ISO 8601-1:2019 and 8601-2:2019. But can I rely on them? They were published in 2016. What has changed between then and the official release in 2019?
Also, neither RFC3339 nor Wikipedia discuss features like open-ended time intervals. They are in the draft, but did they make the final cut? And in what form? No idea...
The closest I've found to "official" documentation of the 2019 update is a short summary of the changes, hosted by Library of Congress[1]. Which is nice to have, but I sure wish the real standard was public.
Indeed. The comprehensive Wikipedia article on ISO8601 is as far as we go
https://en.wikipedia.org/wiki/ISO_8601
Thanks for pointing out rfc3339 - this appears to be the "useful part" of ISO8601 for data interchange.
https://tools.ietf.org/html/rfc3339