ISO 8601-2:2019

This document specifies additional representations of dates of the Gregorian calendar and times based on the 24-hour clock that extend the basic rules and composite elements of those defined in ISO 8601-1. These representations are specified as character strings for use in information interchange. It is also applicable for representing times and time shifts based on Coordinated Universal Time (UTC).
These extensions include:
-  uncertain or approximate dates, or dates with portions unspecified;
-  extended time intervals;
-  divisions of a year;
-  sets and choices of calendar dates;
-  grouped time scale units;
-  repeat rules for recurring time intervals; and
-  date and time arithmetic.
This document excludes the representation of date elements from non-Gregorian calendars, or times not from the 24-hour clock. This document does not address character encoding of representations specified in this document.