[tz] TZDB use cases

[Stephen Colebourne]

On Thu, 30 Sept 2021 at 09:42, Jürgen Appel via tz <tz at iana.org> wrote:
> a) Although possibly a boundary case, I'd like to add that when using location
> based identifiers, for past or future times, a unique conversion from local
> time to UTC is sometimes impossible:
>
> 2020-10-25 02:30:00 Europe/Copenhagen cannot be mapped unambiguously to UTC,
> whereas the reverse mapping works of course. I am not aware of any software
> that takes this into account.

FYI, java.time.* and Joda-Time provide explicit tools for developers
to manage gaps and overlaps on the local timeline.

> Therefore a reasonable requirement from the downstream user side would be to
> convey information about such conditions ('invalid time' , 'ambiguous time',
> evtl. also 'unconfirmed tzdata' for times not covered in the database)

>From my perspective as a software library author, handling of these
two use cases belong at the downstream library/consumer level, not at
the TZDB level.

thanks
Stephen