> Continuing the pedantalization escalation the 21st century started 2001-01-01 if you follow the Gregorian calendar "named after Pope Gregory XIII, who introduced it in October 1582"

In the same spirit of pedantry:

Well, no, Gregory is beside the point; if you are referring to Anno Domini dates, the 21st Century (and Second Millennium) began on the first day of the year A.D. 2001, whether or not you prefer the Gregorian Calendar.

Preference for the Gregorian Calendar or not will affect when you believe A.D. 2001 started, though.

And the transition from Julian to Gregorian just changed the leap year rules. It didn't change the fact that there's no year 0. The 1st Century began in the year 1.

I don't believe anything in ISO 8601 refers to the century starting 2000-01-01 as "the 21st century" (though apparently it does, in some contexts, allow "20" to refer to the century that started on that date).

The century called "the 1900s" overlaps with the 20th century for 99 of 100 years.

> And the transition from Julian to Gregorian just changed the leap year rules. It didn't change the fact that there's no year 0.

It is not issue of Julian vs Gregorian date, but the issue that ISO 8601 uses different conventions. Traditionally, there were two disjoint timelines (AD/CE and BC/BCE), both starting at year 1. Within these conventions, it make sense that first century is 1-100 CE, second century 101-200 CE and so on.

ISO 8601 instead uses astronomical year numbering ( https://en.wikipedia.org/wiki/Astronomical_year_numbering ), which has signed integer year numbers including year 0. Therefore, there is no beginning of timeline and most natural way how to define centuries by div 100, e.g. years 0-99 is century 0, and so on.