PostgreSQL output timestamp with time zone

Question

Trying to output a timestamp with time zone in the PostgreSQL.
I have tried several params, the first two are fine.

(1):

select time at time zone 'UTC' from sf where id='365498691097858048';

it outputs:

2013-08-08 15:44:13

(2):

select time at time zone 'PDT' from sf where id='365498691097858048';

it outputs

2013-08-08 08:44:13

But the third one confuses me:

(3):

select time at time zone  from sf where id='365498691097858048';

2013-08-08 

'UTC-07' is the same as 'PDT', why the outputs are totally different?

Answer 1

This happens because PostgreSQL is being compliant with the POSIX standard when you use timezones without names.

From the IANA Timezone database :

 # Be consistent with POSIX TZ settings in the Zone names, # even though this is the opposite of what many people expect. # POSIX has positive signs west of Greenwich, but many people expect # positive signs east of Greenwich. For example, TZ='Etc/GMT+4' uses # the abbreviation "-04" and corresponds to 4 hours behind UT # (ie west of Greenwich) even though many people would expect it to # mean 4 hours ahead of UT (ie east of Greenwich). 

From the POSIX Spec as documented for GNU libc (emphasis mine):

The offset specifies the time value you must add to the local time to get a Coordinated Universal Time value. It has syntax like [+|-]hh[:mm[:ss]]. This is positive if the local time zone is west of the Prime Meridian and negative if it is east. The hour must be between 0 and 24, and the minute and seconds between 0 and 59.