The usual process would be to write this as an external library first, and then we can consider whether to pull it into the standard library.

That's something we're looking for too knative/eventing#3555

Can https://github.com/rickb777/date be considered the external library first ?!

The parser function is here:
https://github.com/rickb777/date/blob/master/period/parse.go#L39

Some other libraries and software in Go that attempts to parse/output ISO periods:

• https://github.com/dylanmei/iso8601
• https://github.com/ChannelMeter/iso8601duration
• https://github.com/senseyeio/duration
• https://github.com/rickb777/period
• https://github.com/BrianHicks/finch/blob/develop/duration/duration.go#L33
• https://github.com/sosodev/duration

There is an ongoing discussion regarding adding ISO 8601 format support in json/v2 here: #63397 (comment)

Might be worth chiming in over there, perhaps upvoting the comment to signal the level of demand for this feature.