Re: [FEATURE REQUEST] Timezone support in org-mode datestamps and org-agenda

[Tim Cross]

Jean Louis <bugs@gnu.support> writes:

> * Sterling Hooten <hooten@gmail.com> [2023-01-27 09:06]:
>>   Offset
>>         Constant duration difference between times of two time scales
>>         (ISO). i.e., a quantity to combine with a time scale to produce
>>         a wall time. e.g., Nepal uses a +5:45 offset from the UTC time
>>         scale.
>
> I would be careful calling it constant as time offset is changing
> depending of daylight saving time. It is not constant.
>
> Time offset time stamp may be derived from time zone time. I am sure
> about this.
>
> Time zone time stamp cannot be unambiguously derived from time
> offset. I am mostly sure about this.
>
>> What kinds of representations would a calendar system capable of
>> handling timezones require?
>> 
>> • Instant (fixed)
>>   • This is referring to an unambiguous moment in time
>>   • e.g., 2007-02-03T05:00:00.000Z
>> • Offset (fixed)
>>   • This captures the idea of "when did it happen for the person who
>>     made the observation"
>>   • e.g., 2007-02-03T04:00:00.000+01:00
>
> Offset is not that fixed, maybe from viewpoint of storage as maybe it
> is considered fixed in it's representation, but you have to keep in
> mind that time offset by it's definition is changing itself, suddenly,
> depending of daylight saving and time zone.
>

I think your misinterpreting the intent here. If you specify a timestamp
with offset, it is fixed. It does not change with daylight savings or
any other change in rules for a time zone. It does not even specify a
time zone. While it is true that a specific location may have time zone
changes or that the offset from UTC might change as a result of daylight
savings, an offset specified in a times stamp does not change and is
fixed.  This is one of the limitaiton with using offset.

I also think it is a mistake (which I've seen others suggest in this
thread) to equate an offset as being an abbreviation for a time
zone. For example, some have suggested things like +10000 being the same
as AEST and both being abbreviations for Australia/Sydney outside of
daylight savings periods. I think that is incorrect. +1000 is a fixed
offset from UTC and while it may be the same offset used in a time zone
abbreviation like AEST or in a full time zone specification like
Australia/Sydney, it is not a time zone specification, only an offset
for a specific point in time.