Re: [POLL] Proposed syntax for timestamps with time zone info (was: [FEATURE REQUEST] Timezone support in org-mode datestamps and org-agenda)

[Jean Louis <bugs@gnu.support>]

* Tim Cross <theophilusx@gmail.com> [2023-02-01 11:10]:
> I think the confusion relates to context interpretation. If you see
> @Europe/Berlin in isolation, then it is ambiguous as it can refer to
> two different time zone definitions (standard v daylight savings).

Of course, without the time stamp, the time zone alone cannot give
time.

> However, if you consider it in conjunction with a date and time, as
> in 2023-03-23 02:30 @Europe/Berlin, then it isn't ambiguous - in
> that case, it really just says 'Lookup the time zone offset in the
> databse for Berlin as of that date and time.

Exactly that.

> Now that could change - for example, the German government might
> make a temporary or permanent change that would change the offset
> from UTC for that date+time the day after I look at it (or export
> it).

It cannot change in past.

It will not change drastically or capriciously as Germany aligns with
other countries and ISO standard.

It is more likely that ISO non-members deviate from international
coordination of time related definitions:

ISO - Members:
https://www.iso.org/members.html

> Personally, I cannot see the use case of including both a fully
> qualitifed time zone (as in @Europe/Berlin) and an offset, but I
> also don't know all possible use cases - there could well be use
> cases where you want/need to record both the location time zone as
> well as the offset at the point when you recorded the timestamp.

Time Zones vs. Offsets – What's the Difference? Which Is Best?:
https://spin.atomicobject.com/2016/07/06/time-zones-offsets/

Quoting:
--------

- Given a time zone and a UTC time, you can know the offset—and
  therefore the local time.

- Given a local time and an offset, you can know UTC time, but you do
  not know which time zone you’re in (because multiple timezones have
  the same offset).

- Given UTC and an offset, you can know the local time.

- Given a time zone and an offset, you don’t know much. 

- That’s why a calendar systems work with time zones, offsets, and
  UTC; 

- we need the offset to go from local time to UTC, and we need the
  time zone to go from UTC to local time.

-- 
Jean

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