Javascript: Get client's TimeZone

How to know client's time and time zone? The simplest way that can do this is using the javascript.

<script type="text/javascript">
function displayTime()
{
  var localTime = new Date();
  //this one will give you the GMT offset
  var timezone = localTime.getTimezoneOffset()/60 * (-1);
  var div = document.getElementById("div1");
  div.innerText = "GMT" + timezone;
}
</script>

Or simply use

<script type="text/javascript">
function displayTime()
{
  var div = document.getElementById("div1");
  div.innerText = (new Date()).toString();
}
</script>