> which does not match calendar year everywhere

Yes, this is the reason that ISO-8601 ranges are not sufficient and a Quarter indicator is necessary.

The problem is that if you carry the string '2021-Q1' across countries borders, the days it represents change. That makes for a very bad time interchange format.

I agree that this feature makes quarter indicator necessary. But it also makes it required that the string is something more complex.

If you carry the string '2021-10-24' across countries borders, the moments in time that it represents change as well.

That makes doesn't make it a bad time interchange format for certain use cases, though. Just like everything else with time, one must be aware of caveats. Those include timezones, leap periods, culture, DST, formats, legislation (modern and ancient), etc.

But it overlaps. It's off, but less than 100% off.

"2021-Q1" can be a completely different and non-overlapping range between regions, and even between companies in the same region.

2021-10-24 in UTC+14 (Line Islands) and UTC-10 (Hawaii) are, in fact, 100% off.

Yeah the Line Islands are the exception as always.

We could've gone with most of New Zealand (UTC+13 summer time) and any of the US territories in UTC-11. These are marginal cases, not exceptional ones.

Fine, they overlap with the UTC day. They are less than 100% compared to the UTC day, even if they can be 100% off with each other.