BUG #14660: Conversion to UTC not properly computed

The following bug has been logged on the website:

Bug reference: 14660
Logged by: Todd Brandys
Email address: tbrandys(at)webcellence(dot)com
PostgreSQL version: 9.6.3
Operating system: Slackware Linux
Description:

According to
https://www.postgresql.org/docs/9.6/static/datatype-datetime.html

Conversions between timestamp without time zone and timestamp with time zone
normally assume that the timestamp without time zone value should be taken
or given as timezone local time. A different time zone can be specified for
the conversion using AT TIME ZONE.

With this in mind, the following output is troublesome:

vendita=# select now()::timestamp(0), now()::timestamp(0) with time zone,
timezone('utc', now()::timestamp(0)), timezone('utc', now()::timestamp(0)
with time zone);
now | now | timezone |
timezone
---------------------+------------------------+------------------------+---------------------
2017-05-18 08:55:10 | 2017-05-18 08:55:10-07 | 2017-05-18 01:55:10-07 |
2017-05-18 15:55:10
(1 row)

vendita=# show timezone;
TimeZone
------------
US/Arizona
(1 row)

The computation where the assumed use of the local time zone should occur is
incorrect (the third column).