Earlier quoted context omitted.
> ISO 8601 does NOT work with future dates. It does not work with cross border appointment booking. Why is this the case? Is it some time zone shenanigans?
Timezones change, more frequently than you realize. Some countries even used to change when summer time started regularly. It's got better than it used to be afaik. Also you need to take into account where the user is going to be. With future dates, time is relative. So at the very least you often need the time + the timezone of the location. Book me a table at 8, means book me a table in Berlin time at 8, not in San…
You can represent time zones with that format. So long as you have a source time zone, target time zone and tzdata you can convert any time accounting for all the particularities of any particular zone.