PostgreSQL – how to render date in different time zone?

The key is to switch the local timezone to the desired display timezone, for the duration of the transaction:

begin;
set local timezone to 'EST5EDT';
select to_char('2012-05-29 15:00:00'::timestamp at time zone 'CDT',
  'YYYY-MM-DD HH24:MI:SS TZ');
end;

The result is:

2012-05-29 16:00:00 EDT

Note that with set [local] timezone it is required to use full time zone names instead of abbreviations (for instance, CST would not work). Look up in the pg_timezone_names view for valid choices.

To use that method in a context similar to a to_char() call, I believe this function does the job:

CREATE FUNCTION display_in_other_tz(
      in_t timestamptz,
      in_tzname text,
      in_fmt text) RETURNS text
AS $$
DECLARE
 v text;
 save_tz text;
BEGIN
  SHOW timezone into save_tz;
  EXECUTE 'SET local timezone to ' || quote_literal(in_tzname);
  SELECT to_char(in_t, in_fmt) INTO v;
  EXECUTE 'SET local timezone to ' || quote_literal(save_tz);
  RETURN v;
END;
$$ language plpgsql;