PostgreSQL time zones - postgresql

Question

I'm trying to understand how the session time zone works in PostgreSQL.
When I set session time zone to '01:00', returned date is in '-01:00'. Sign is always inverted and I do not understand this behavior.
How can this be explained?
postgres=# set time zone '00:00';
SET
postgres=# select timestamp with time zone '2017-11-29 15:00:00.000000+00';
timestamptz
------------------------
2017-11-29 15:00:00+00
(1 row)
postgres=# set time zone '01:00';
SET
postgres=# select timestamp with time zone '2017-11-29 15:00:00.000000+00';
timestamptz
------------------------
2017-11-29 14:00:00-01
(1 row)
postgres=# set time zone '-01:00';
SET
postgres=# select timestamp with time zone '2017-11-29 15:00:00.000000+00';
timestamptz
------------------------
2017-11-29 16:00:00+01
(1 row)

Answer 1

example, here
ts is time (text) - just time as we say it, :
not_aware the time if we don't know its timezone (local to the saying person)
aware the time, but knowing the timezone of it(if server time zone is UTC, then of utc zone)
, so:
s=# with s(city,tz) as (values('Moscow','UTC+3'),('New York','UTC-5'))
, ts as (select '2017-01-01 00:00:00'::text ts)
select *,ts::timestamp at time zone tz not_aware, ts::timestamptz at time zone tz aware
from s
join ts on true;
city | tz | ts | not_aware | aware
----------+-------+---------------------+------------------------+---------------------
Moscow | UTC+3 | 2017-01-01 00:00:00 | 2017-01-01 03:00:00+00 | 2016-12-31 21:00:00
New York | UTC-5 | 2017-01-01 00:00:00 | 2016-12-31 19:00:00+00 | 2017-01-01 05:00:00
(2 rows)
lets say you you come from Lodon and have have watches that show UTC0 time
- local for London (this is your server with timezone UTC0). Now are fly to Moscow, looking at the Kremlin Clock and you see the ts, midnight. Your watches are aware of time difference and they show you that to Ney Year is in three hours (this is timestamptz - aware column). This is why UTC+3 requires to minus the time. Because for your server Moscow time is in future, thus when you see local 2017, for server it is still 2016 and gonna be like it for +3 hours...
Now the behaviour that you expected (not_aware column) is more tricky. The server runs at UTC0, but has to pretend it know nothing about the "real time". As timestamp without time zone is such type. So it behaves as if you were watching new year celebration on Red Square on TV and would just scroll the bar to adjust your watches to Kremlin Clock, so in fact the time you see is your London time +3 (not_aware) column.

Answer 2

I came across this while looking for answers to other questions.
In case you never figured it out:
I think this is a quirk of the timezone parsing code trying to follow POSIX syntax, as noted above. You get the same behavior by e.g. SET TIME ZONE 'CET1:00'; or SET TIME ZONE 'CET1'.
I think the "actual" SQL way of saying what you were trying to say is SET TIME ZONE 1.
You probably don't want to have this be set as your timezone anyhow because it doesn't track DST changes. You probably want to choose a timezone by name from the tz database, for example America/New_York or (going with the GMT+1 example) Europe/Madrid.