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

[Jean Louis <bugs@gnu.support>]

Dear Heinz,

Thanks, let me see.

* Heinz Tuechler <tuechler@gmx.at> [2023-01-31 01:02]:
> Dear Jean Louis,
> 
> it appears to me that you mix two aspects. I agree with you that a time
> zone needs an offset from UTC to be defined. Consequently the definition
> of a time zone requires an offset.

Yes, that is good our mutual understanding.

> But an offset does not need a time zone to define a time.

I am trying to understand what you wanted to say with the
above. 

Before representing time with the UTC offset:
---------------------------------------------

To derive the representation of time with the UTC offset, one needs
time zone, as UTC offset is defined in the time zone. That is what you
also said above.

After representing time with the UTC offset:
--------------------------------------------

That time is defined, and at that time point does not need time zone. 

I am not concerned of time representation after UTC offset has been
derived, but of programming calculations to users' local time, as for
example in Org Agenda.

> For example, true mean local solar time of a specific longitude can
> be described by UTC plus offset.

Solar time - Wikipedia:
https://en.wikipedia.org/wiki/Solar_time

By meaning that solar time should be related to longitude, I am
totally with you Heinz. It is also true that one could disregard the
definition of UTC offset from the political reality, and calculate it
absolutely.

That condition I have already mentioned, when I said, that means we
are making "new type of time" in Org, if we start calculating it that
way. 

The meaning of "UTC offset" is however, political. Please see the UTC
offsets here:
https://upload.wikimedia.org/wikipedia/commons/8/88/World_Time_Zones_Map.png

Look at the map, find Kazakhstan with UTC offset +6 on the same
longitude with Russian Federation with UTC offset +5.

Observe Kazakhstan with UTC offset +6 on the same longitude with China
with the UTC offset +8.

That alone should tell you that solar time is not really related to
UTC offset, but we could say it is "approximate" with few hours more
or less.

Of course you can describe solar time with UTC offset, but do not
assume it will be accurate.

> I agree with your criticism of "They refer to local *solar time* at a
> particular place." This is written in
> https://en.wikipedia.org/wiki/UTC_offset , but not even there the
> description is consistent.

We can say it is approximate what they mean.

> Of course, for every finite offset, we can find a corresponding
> particular place (a longitude).

I wish it would be so, but it is not so. It is approximate, just look
at the map.

And please tell me if after this you still think there is something
wrong?

-- 
Jean

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