postgresql timezone conversion 'EST' != '-05'

Question

I am running PostgreSQL 9.6.6 on x86_64-pc-linux-gnu and my time zone is set to \'UTC\'.

Does anyone know why the results of the following SELECT statem

Answer 1

Okay I think I found an answer to my own question:

According to the PostgreSQL docs, section 9.9.3 at the following link https://www.postgresql.org/docs/9.6/static/functions-datetime.html

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name can be specified in any of the ways described in Section 8.5.3.

So using the INTERVAL syntax, the following appears to work:

SELECT timezone(INTERVAL '-05:00', '2017-12-21');
      timezone       
---------------------
2017-12-20 19:00:00

I think it is still curious, what exactly SELECT timezone('-05', '2017-12-21'); means, as the following also provides the expected result (with the addition of a TZ offset):

SELECT timezone('-05', '2017-12-21'::timestamp);
        timezone        
------------------------
2017-12-20 19:00:00+00