[tz] TZDB use cases

[Guy Harris]

On Oct 2, 2021, at 3:02 PM, Stephen Colebourne via tz <tz at iana.org> wrote:

> The point above is documenting how things *are*. Business applications
> and calendaring systems really do store the local date/time and zone
> ID from TZDB for future events. This works precisely because there are
> enough IDs provided by TZDB to make this work.

If we're talking about *future* events, the only IDs that would be required to make this work would be IDs corresponding to regions that differ in dates and times after the earliest date and time that matters to the business application in question.  And any business application (I'm including calendaring systems in the category of "business application" here) that uses the tzdb couldn't have done so before the tzdb was first released (which was 14-15 years after 1970) - an application could not have, in 1963, have recorded an event scheduled for 1965 as a date/time and zone ID from tzdb.

So it's not as if there was an *inherent* need for Europe/Oslo as a tzdb ID to make these applications work; if there had never been a Europe/Oslo ID, and Norwegian locations had used Europe/Berlin, that would have been sufficient to, for example, record future events taking place in Norway.

The reason why we would continue to have a Europe/Oslo ID even if we put Norway into the Europe/Berlin tzdb region is for *backwards compatibility* with applications that have, for example, stored future events with a tzdb ID of Europe/Oslo.