A self-contained full implementation of ISO 8601 is of dubious value anyway.

Whether time is used for a private application or for a standard, the important part is the agreement on what information is being conveyed and in what format. Receiving just the week of the year when you were expecting a time range is not useful.

Likewise, having some sort of official support for "20" as identifying the century (say via a Century type) is a lot of code to carry around and maintain - especially before there's a business need to represent centuries. You tend to need full date time instants, then maybe dates and times independently, then maybe durations - with only a few applications tending to need things like ranges.

Yeah, this means that when you want to use ranges and choose ISO 8601 as your format, you may not find readily-available, off-the-shelf parsers.