TIMEZONE

Returns a timestamp for the specified time zone and timestamp value.

TIMEZONE ('timezone', { timestamp | timestamptz )

Specify the time zone as a UTC abbreviation or an IANA time zone name. For example:

'GMT'
'PDT'
'Singapore'
'US/Pacific'

The second argument must be a timestamp (with or without time zone).

For example:

premdb=# select localtimestamp, timezone('GMT', localtimestamp);
         timestamp          |           timezone            
----------------------------+-------------------------------
 2017-11-21 16:56:11.649456 | 2017-11-21 08:56:11.649456-08
(1 row)

premdb=# select localtimestamp, timezone('US/Pacific', localtimestamp);
         timestamp          |           timezone            
----------------------------+-------------------------------
 2017-11-21 16:56:24.481238 | 2017-11-21 16:56:24.481238-08
(1 row)