ODF 1.4 part 3 sect. 19.341 number:calendar - correct Julian to Gregorian - correct ISO ref

In Part 3, 1.3 Normative References , and in Part 4, 1.4 Normative References : [ISO8601] ISO 8601:2019 Date and time — Representations for information interchange — Part 1: Basic rules, International Organization for Standardization, 2019 In Part 3, 19.341 number:calendar : gregorian : Gregorian calendar, as defined in [ISO8601] , Section 4.2.1, usually specified for dates no earlier than 1582-10-15. If this calendar is specified for a date earlier than 1582-10-15, the meaning is implementation-defined. In Part 4, 7.4 Year 1583 : Evaluators claiming to implement “Year 1583” can calculate dates correctly starting from the January 1 of the (ISO) year 1583. This means that the evaluator correctly determines that 1900 was not a leap year, and can handle year values for dates back to at least 1583. These calculations use the ISO (Gregorian) calendar specified by [ISO8601] , that is, the calculations use the usual rules for the ISO (Gregorian) calendar, regardless of locale. This calendar began official use in some locales in 1582, but other locales used other calendars (such as the Julian calendar) and switched to the Gregorian calendar at different times in history, if they switched at all. Evaluators may choose to support dates before the official introduction of the Gregorian calendar on 15 October 1582 and for years even earlier than this; such evaluators may use a proleptic Gregorian system (continuing the dates backwards from 15 October 1582 as if the Gregorian calendar existed for those dates), but the use of such a system is implementation-defined. Note that the ISO (Gregorian) calendar has never been and is still not currently in universal use. Correct date calculations in this calendar system require that leap years be handled correctly. In this calendar system, leap years include 29 days in February (which otherwise has 28 days), for 366 total days in a leap year. In general, all years evenly divisible by 4 are leap years. However, years that are divisible by 100 shall also be divisible by 400 to be a leap year; otherwise, they are common (non-leap) years.