BUG #15388: time convert error when use AT TIME ZONE '+8'

The following bug has been logged on the website:

Bug reference:      15388
Logged by:          hao.hu
Email address:      huhao0715@gmail.com
PostgreSQL version: 10.1
Operating system:   linux
Description:

1. What did you do?
SET TIME ZONE 'utc';
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE '+8';

2. What did you expect to see?
        timezone
------------------------
 2001-02-16 12:38:40+00

3. What did you see instead?
        timezone
------------------------
 2001-02-17 04:38:40+00