title: Oracle change dbtimezone
body: <p>My database was configured with an <code>dbtimezone=+2:00</code>:</p>
  
  <p>When my application sends a date which has a timezone, does Oracle automatically translate the date to its <code>dbtimezone</code> and store it in the column?</p>
  
  <p>When my application asks for a field date, does Oracle automatically translate it to the application timezone?</p>
  
  <p>In order to be consistency with business rules, I wanted to change this <code>dbtimezone</code> to UTC. So I made the <code>alter database set time_zone='UTC'</code> command, I restarted the oracle server and now the <code>select dbtimezone from dual;</code> command returns "UTC".</p>
  
  <p>But, all fields date in DB haven't changed (no change -2 hours from GMT+2 to UTC). When I ask the sysdate, it returns the GMT+2 date ... I try to change my SQL Developer configuration timezone to UTC but it didn't change anything. Do I have an issue of Oracle session parameters that convert my DB data to GMT+2 before displaying it ?</p>
  
  <p>Finally, does anyone have a good practice to make this change ? (change the database timezone and existing date to a new timezone).</p>