Thread: why at time zone tz_offset not equal tz_name? tz_name has same offset in pg_timezone_names

H Witt <i5d@live.com> writes:
> select
> '2025-02-03 15:04:05'::timestamptz at time zone '+08:00',
> '2025-02-03 15:04:05'::timestamptz at time zone 'Asia/Shanghai'

They're different because '+08:00' is read as a POSIX time zone
specification, which has the opposite sign convention to what
you are thinking.  See

https://www.postgresql.org/docs/current/datatype-datetime.html#DATATYPE-TIMEZONES

and especially

https://www.postgresql.org/docs/current/datetime-posix-timezone-specs.html

and remember the adage "The great thing about standards is there are
so many to choose from".  Sadly, with only two relevant standards
humanity has still managed to cover the design space of which
direction from Greenwich gets the positive sign.

            regards, tom lane

On Tue, Apr 29, 2025 at 02:40:45AM +0000, H Witt wrote:
> select 
> '2025-02-03 15:04:05'::timestamptz at time zone '+08:00',
> '2025-02-03 15:04:05'::timestamptz at time zone 'Asia/Shanghai'
> 
> display
> |2025-02-02 23:04:05.000|2025-02-03 15:04:05.000|

The sign is wrong in the first column:

    SELECT
    '2025-02-03 15:04:05'::timestamptz at time zone '-08:00',
    '2025-02-03 15:04:05'::timestamptz at time zone 'Asia/Shanghai'
    ;
          timezone       |      timezone
    ---------------------+---------------------
     2025-02-04 04:04:05 | 2025-02-04 04:04:05

See:

    https://www.postgresql.org/docs/current/datetime-posix-timezone-specs.html

    The offset fields specify the hours, and optionally minutes and
    seconds, difference from UTC. They have the format hh[:mm[:ss]]
    optionally with a leading sign (+ or -). The positive sign is used
    for zones west of Greenwich. (Note that this is the opposite of
    the ISO-8601 sign convention used elsewhere in PostgreSQL.) hh
    can have one or two digits; mm and ss (if used) must have two.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Do not let urgent matters crowd out time for investment in the future.