use your code within
<script type="text/javascript">
function window.onload()
{
var d = new Date()
var gmtOffSet = -d.getTimezoneOffset();
var gmtHours = Math.floor(gmtOffSet / 60);
var GMTMin = Math.abs(gmtOffSet % 60);
var dot = ".";
var retVal = "" + gmtHours + dot + GMTMin;
document.getElementById('<%= offSet.ClientID%>').value = retVal;
}
</script>