(Not suggesting that relying on the free draft is great, just providing a reference for people interested in the actual content of ISO 8601, rather than the principles around this issue. The spec is a good read for people interested in datetime representations.)
https://www.loc.gov/standards/datetime/iso-tc154-wg5_n0038_i...
https://www.loc.gov/standards/datetime/iso-tc154-wg5_n0039_i...
Do take note of the caveats on the cover.
(Not suggesting that relying on the free draft is great, just providing a reference for people interested in the actual content of ISO 8601, rather than the principles around this issue. The spec is a good read for people interested in datetime representations.)