timezone abbreviation in timestamp string input

When a timestamp string input contains a timezone abbreviation (CDT,
PST, etc), which timezone offset is used? The input date's or today
date's? The result on my computer suggests the latter.

# create table ts (ts timestamptz);
# insert into ts values ('2004-10-17 00:00:00 CDT'); -- UTC-5
# insert into ts values ('2004-11-17 00:00:00 CDT'); -- UTC-6
# select ts at time zone 'utc' from ts;
       timezone
---------------------
  2004-10-17 05:00:00
  2004-11-17 05:00:00
(2 rows)

If this is true, then perhaps forbid timezone abbreviation in input
string, or emit warning about this?

--
dave