title: how to get timezone offset value
body: <p>My requirement is , i need to convert the time zone value to offset value 
  for ex.. Asia/Dubai to +4 .. 
  consider am intializing date in js. </p>
  
  <pre><code>var currentDate = new Date();
  </code></pre>
  
  <p>and it is possible to set the time zone and get the offset..</p>
  
  <pre><code>currentDate.setTimezone("Asia/Dubai");
  var offsetValue=currentDate.getTimezoneOffset();
  
  alert('offsetValue---'+offsetValue);
  </code></pre>
  
  <p>its not working.. page error at set time zone.. is there any other way to get the offet value?
  Need to consider DST too..</p>