Postgres 15

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set time zone 'UTC';
create table val ( tsval timestamp );
insert into val values ('2024-01-01 10:42:13');

SET

CREATE TABLE

INSERT 0 1

show time zone;
select tsval at time zone 'UTC', tsval at time zone 'Australia/Sydney', tsval
from val;

TimeZone

UTC

SHOW

timezone
timezone
tsval

2024-01-01 10:42:13+00
2023-12-31 23:42:13+00
2024-01-01 10:42:13

SELECT 1

alter table val alter column tsval type timestamptz;

ALTER TABLE

show time zone;
select tsval at time zone 'UTC', tsval at time zone 'Australia/Sydney', tsval
from val;

TimeZone

UTC

SHOW

timezone
timezone
tsval

2024-01-01 10:42:13
2024-01-01 21:42:13
2024-01-01 10:42:13+00

SELECT 1