I think the main usage of ISO 8601 was to represent a point in time in UTC for exchange, not a local time plus timezone. When a date such as 2021-02-26 18:38:00

I think the main usage of ISO 8601 was to represent a point in time in UTC for exchange, not a local time plus timezone. When a date such as 2021-02-26 18:38:00+0900 is provided, it did not meant 2021-02-26 18:38:00 in Asia/Tokyo, but meant 2021-02-26 09:38:00 in UTC. e.g., 09:38:00Z and 18:38:00+0900 and 16:38:00+0700 were supposed to be equivalent. In this case, timezone is external to this format and the timestamp can be calculate back to UTC without relying on zoneinfo (which is very useful for exchange). Any further conversation would be external to this format. e.g., in case of calendar, it's probably more useful to have datetime in local timezone rather than of the other party's timezone so the tuple would be (utc_time, local_tz) rather than (other_localtime, other_tz, local_tz).