> ... billed as “an extension to the ISO 8601 / RFC 3339” formats

The quote comes from MDN[1], but is wrong on so many levels:

1. RFC 3339 is not a format; it is a meta-format. It specifies properties that any timestring used in internet applications should have. One application could require that "T" be used to separate the date and the time, while another could require a space to be used, and both applications would conform to 3339. Using the same parser to parse times and dates from the two applications would be incorrect, as "1234-12-12 12:34 ..." unambiguously represents the date of December 12, 1234 in the former but the time of 12:34 on the same day in the latter.

2. RFC 3339 is thus not a subset of ISO-8601; there exists a subset of ISO-8601 which satisfies RFC 3339, but there also exist many timestrings that are not subsets of ISO-8601 which also satisfy it (most famously replacing "T" with a space, but that is merely an example for which RFC 3339 conforming applications may deviate from the ABNF in the RFC for readability).

3. It is obvious from both #2 and the table in TFA that RFC 9557 can't be an extension of ISO-8601 given that there are valid ISO-8601 timestrings in the table that are invalid RFC 9557 timestrings.

4. This is a minor nitpick, bu RFC 9557 also alters the semantics of some RFC 3339 timestrings with regards to offsets. RFC 3339 specified "Z" (or "z") for the offset being the same as +00:00, while 9557 has "Z" being the same as -00:00; the meat of RFC 9557 is adding suffix-tags to RFC 3339, so I wouldn't quibble with calling 9557 an extension of 3339.

1: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...