POSIX TS spec reverses the meaning of TZ offset compared to ISO (was: [POLL] Proposed syntax for timestamps with time zone info (was: [FEATURE REQUEST] Timezone support in org-mode datestamps and org-agenda))

[Ihor Radchenko <yantar92@posteo.net>]

[ adding Org ML back to CC ]

Christian Moe <mail@christianmoe.com> writes:

>> Note, however, that because we are conforming to POSIX TZ, @UTC+2 is two
>> hours _behind_ the Greenwich.
>
> Ouch.

This is probably something we need to discuss further.

Dear All,

There is potential confusion coming from the different interpretations
of the time zone offsets used in ISO8601 and POSIX TZ specs:

https://en.wikipedia.org/wiki/ISO_8601#Time_zone_designators:

    Negative UTC offsets describe a time zone west of UTC±00:00, where
    the civil time is behind (or earlier) than UTC so the zone
    designator will look like "−03:00","−0300", or "−03".

https://www.di-mgt.com.au/wclock/help/wclo_tzexplain.html:    

    [Asia/Beijing]
    TZ=CST-8

    Minutes and seconds are optional, so CST-8 and CST-08:00:00 mean the
    same thing. Note that the sign convention (+/-) used in a Posix TZ
    string is the opposite to that used in Internet time offsets (RFC
    3339) and in Arthur David Olson's TZ data files.

So, the equivalent time zone offsets in, say, Singapore will be:

[2023-02-01 12:00 @Asia/Singapore]
[2023-02-01 12:00+08] ;; local time 8 hours earlier than UTC as in ISO8601 spec
[2023-02-01 12:00 @UTC-08] ;; UTC is 8 hours earlier than local time as in POSIX TZ spec

This is potentially confusing.

We may, in theory, reverse the meaning of @UTC[+-]NN manually, but then
values like @EST+5 will remain reversed, again creating the confusion.
Or, if we reverse everything in @..., users more familiar with POSIX TZ
will get confused.

Any ideas?

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
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