Bug in AT TIME ZONE contruct between EST and INTERVAL '-05:00'

When selecting with the AT TIME ZONE or timezone function, the returned value
is not always a timestamp. This is inconsistent with the documentation.

To reproduce this problem, run these two queries.

SELECT now() AT TIME ZONE 'EST'

returns 2003-03-13 21:27:14.63401-05

SELECT now() AT TIME ZONE INTERVAL '-05:00'

returns 1167 days 21:54:30.952135995

(Queries were run at different time, the problem is that it is returning an
interval instead of a timestamp).

When I attempt to cast the interval as a timestamp it gives me an error saying
that it's not possible.

I am pretty sure this is a bug. Following is supoprting dpcumentation from
the manual.

From the documentation

In these expressions, the desired time zone can be specified either as a text
string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00').

Examples (supposing that TimeZone is PST8PDT):

SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'MST';
Result: 2001-02-16 19:38:40-08

SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'MST';
Result: 2001-02-16 18:38:40

The first example takes a zone-less timestamp and interprets it as MST time
(GMT-7) to produce a UTC timestamp, which is then rotated to PST (GMT-8) for
display. The second example takes a timestamp specified in EST (GMT-5) and
converts it to local time in MST (GMT-7).

The function timezone(zone, timestamp) is equivalent to the SQL-compliant
construct timestamp AT TIME ZONE zone.

Josh.