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.
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.
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.