This is the module for CalendarDate and CalendarDateTime in the ISO 8601 calendar. For every date the ISO calendar is identical to Gregorian - the same months, the same leap year rule and the same 15 October 1582 cut-off - and it reuses the Gregorian types directly. The two differ only in how week dates are numbered: ISO weeks start on Monday and week 1 is the first week containing at least four days of the new year (equivalently, the week holding that year's first Thursday). Use this module's fromWeekDate for ISO-8601 week numbering; the Gregorian module provides the alternative Sunday-based, at-least-one-day rule.