This is a misconception. ISO8601 has six versions, is 14 pages long, and defines way to many ways to represent dates and times.

Everyone should follow RFC 3339.

https://www.rfc-editor.org/rfc/rfc3339#section-5.6

Thanks! Wasn't aware of that spec.

But they‘re mostly trivial variations of each other, or durations that nobody uses at all.

That's the problem though. If you don't support the durations that nobody uses, then you can't fully support ISO8601 dates.

Every trivial variation is another if/switch statement.

Durations are not used in places that need a timestamp, and visa versa. More applications need timestamps than need durations.