title: Oracle sql timezone issue
body: <p>My requirement here is to get time in GMT/UTC from a date type column. But when I use cast to cast date to timestamp, it is using US/Pacific timezone as reference though session timezone is set to GMT. So unless I use from_tz, I am not seeing desired result. Is there any other timezone setting in oracle sql that I need to modify to take GMT as reference always?</p>
  
  <pre><code>alter session set time_zone='+00:00';
  select sessiontimezone from dual;
  select current_timestamp from dual;
  select sys_extract_utc(cast (sysdate as timestamp)) from dual;
  select sys_extract_utc(from_tz(cast (sysdate as timestamp), '-07:00')) from dual;
  select sys_extract_utc(current_timestamp) from dual;
  
  
  Session altered.
  
  
  SESSIONTIMEZONE
  ---------------------------------------------------------------------------
  +00:00
  
  
  CURRENT_TIMESTAMP
  ---------------------------------------------------------------------------
  11-APR-16 08.46.42.292173 AM +00:00
  
  
  SYS_EXTRACT_UTC(CAST(SYSDATEASTIMESTAMP))
  ---------------------------------------------------------------------------
  11-APR-16 01.46.42.000000 AM
  
  SYS_EXTRACT_UTC(FROM_TZ(CAST(SYSDATEASTIMESTAMP),'-07:00'))
  ---------------------------------------------------------------------------
  11-APR-16 08.46.42.000000 AM
  
  
  SYS_EXTRACT_UTC(CURRENT_TIMESTAMP)
  ---------------------------------------------------------------------------
  11-APR-16 08.46.42.295310 AM
  </code></pre>
  
  <p>Tasks table has a date type column called task_started. I am looking to get UTC time from this date field. As part of that I was trying to alter session timezone to GMT while inserting the data so that I can simply cast it back to timestamp which is not working.</p>
  
  <pre><code>select task_started from tasks where rownum &lt;2;
  
  TASK_STAR
  ---------
  10-APR-16
  
  desc tasks;
   Name                                      Null?    Type
   ----------------------------------------- -------- ----------------------------
  ...
   TASK_STARTED                                       DATE
  ...
  </code></pre>