BUG #12460: Extracting epoch from timestamp string incorrectly assumes "with time zone"

The following bug has been logged on the website:

Bug reference:      12460
Logged by:          Alan Edel
Email address:      adeconsulting@pacbell.net
PostgreSQL version: 9.3.5
Operating system:   FreeBSD 9.3
Description:

8.3.6 queries using "timestamp" and "timestamptz" casting both give the same
result:

select extract(epoch from cast('2015/01/07 14:00:00' as timestamp));
 date_part
------------
 1420668000
(1 row)

select extract(epoch from cast('2015/01/07 14:00:00' as timestamptz));
 date_part
------------
 1420668000
(1 row)

However, 9.3.5 gives different results, the first one appears to be
incorrect:

select extract(epoch from cast('2015/01/07 14:00:00' as timestamp));
 date_part
------------
 1420639200
(1 row)

select extract(epoch from cast('2015/01/07 14:00:00' as timestamptz));
 date_part
------------
 1420668000
(1 row)