JSR-310 contains lesser-useful fragments of ISO 8601: MonthDay, Period, Year, YearMonth, Month. They might be useful if they are agnostic to calendar systems, but they are actually just for ISO 8601.

Period is doubly problematic. It is important to realize that ISO 8601 is the standard for interchange formats, not the standard for computer data types (compare to, say, IEEE 754). As a result you need additional information to make it a proper data type standard. For ISO 8601 period you would need the actual algorithm to add it to a time point. What happens if you add 1 month (P1M) to January 30, 2021 (2021-01-30)? Would it be 2021-02-28 or 2021-03-02? Should we give both options to end users? You need a set of pluggable policies to make it work. JSR-310 Period class is pretty lacking in this regard. (In fact, pretty much every implementation of "relative delta" has the same problem. It can't be readily used in the business logic.)