Postgres AT TIME ZONE function shows wrong time? - postgresql

Question

I am using transformation to new timezone UTC+3 which is equal to EAT timezone, but Postgres (9.1) shows the wrong time
select '2015-01-13 08:40:00.0'::timestamp with time zone AT TIME ZONE 'UTC+03',
'2015-01-13 08:40:00.0'::timestamp with time zone AT TIME ZONE 'EAT';
(default timezone is Stockholm here)
The result is
"2015-01-13 04:40:00",
"2015-01-13 10:40:00"
Why?
it should be 2015-01-13 10:40:00
if using JodaTime with both timezones then it shows the same correct result '2015-01-13 10:40:00'.

Answer 1

From the Postgres documentation there is the option to use ::timestamptz instead of ::timestamp WITH TIME ZONE and I found preferred results when making the conversion; as it is the most concise of the available options while still being readable.
SELECT created_at
,created_at::timestamp AT TIME ZONE 'EDT' -- yields bad result
,created_at::timestamp WITH TIME ZONE AT TIME ZONE 'EDT'
,created_at AT TIME ZONE 'UTC' AT TIME ZONE 'EDT'
,created_at::timestamptz AT TIME ZONE 'EDT'
2019-03-29 18:49:25.250431 -- raw UTC data
2019-03-29 22:49:25.250431 -- erroneous result
2019-03-29 14:49:25.250431 -- accurate result
2019-03-29 14:49:25.250431 -- accurate result
2019-03-29 14:49:25.250431 -- accurate result

Answer 2

I had similar problem, it gave me the wrong date and time but this answer here gave me a clear understanding and fixed my problem. PostgreSQL wrong converting from timestamp without time zone to timestamp with time zone
So what I did was changing from
SELECT timestamp AT TIME ZONE '+08' FROM orders;
to
SELECT timestamp AT TIME ZONE 'UTC' AT TIME ZONE '+08' FROM orders;

Answer 3

A time zone name spelled like 'UTC+3:00' is a POSIX time zone specification.
In this style, zones west of GMT have a positive sign and those east have a negative sign in their name (e.g "Etc/GMT-14" is 14 hours ahead/east of GMT.)
See http://www.postgresql.org/docs/9.3/static/datatype-datetime.html#DATATYPE-TIMEZONES

Answer 4

EAT(East Africa Time) is three hours ahead of UTC (UTC+03:00).
It's represented as UTC+03:00 in ISO-8601 format.
But AT TIME ZONE only supports timezones represented in POSIX-style.
In POSIX-style, 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.)
Format
PST
ART
W←UTC→E
EAT
HKT
ISO-8601
UTC-08
UTC-03
UTC+00
UTC+03
UTC+08
POSIX-style
UTC+08
UTC+03
UTC+00
UTC-03
UTC-08
In POSIX-style, the correct representation of EAT can be:
'UTC-03'
'UTC-3'
'-3'
-3
The result of the following SQL statement must be TRUE.
select now() at time zone -3 = now() at time zone 'EAT';
Confusingly, set time zone statement supports ISO-8601, not POSIX-style.
Execute the following SQL statements in sequence to find the difference.
#
SQL
Result
①
set time zone -3;
SET
②
select now();
2022-02-24 04:53:41.921391-03
③
select timestamp '2022-02-24 12:00' at time zone -3;
2022-02-24 06:00:00-03
④
select timestamp '2022-02-24 12:00' at time zone +3;
2022-02-24 12:00:00-03
⑤
set time zone 3;
SET
⑥
select now();
2022-02-24 10:54:10.283953+03
⑦
select timestamp '2022-02-24 12:00' at time zone -3;
2022-02-24 12:00:00+03
⑧
select timestamp '2022-02-24 12:00' at time zone +3;
2022-02-24 18:00:00+03
⑨
set time zone local;
SET
⑩
select now();
2022-02-24 15:54:51.79643+08
⑪
select now() at time zone -3;
2022-02-24 10:55:02.209886
⑫
select now() at time zone 3;
2022-02-24 04:55:09.498461
⚠️Please be careful about this.
References:
AT TIME ZONE
Time Zones
POSIX Time Zone Specifications
SET TIME ZONE