function calculate_time_zone() {
   if(document.getElementById('timezone').options == null) return;
   var rightNow = new Date();
   var jan1 = new Date(rightNow.getFullYear(), 0, 1, 0, 0, 0, 0);  // jan 1st
   var june1 = new Date(rightNow.getFullYear(), 6, 1, 0, 0, 0, 0); // june 1st
   var temp = jan1.toGMTString();
   var jan2 = new Date(temp.substring(0, temp.lastIndexOf(" ")-1));
   temp = june1.toGMTString();
   var june2 = new Date(temp.substring(0, temp.lastIndexOf(" ")-1));
   var std_time_offset = (jan1 - jan2) / (1000 * 60 * 60);
   var daylight_time_offset = (june1 - june2) / (1000 * 60 * 60);
   var dst;
   if (std_time_offset == daylight_time_offset) {
	    dst = "0"; // daylight savings time is NOT observed
   } else {
	    // positive is southern, negative is northern hemisphere
	    var hemisphere = std_time_offset - daylight_time_offset;
	    if (hemisphere >= 0)
		    std_time_offset = daylight_time_offset;
	    dst = "1"; // daylight savings time is observed
   }
   var i;
   for (i = 0; i < document.getElementById('timezone').options.length; i++) {
	    if (document.getElementById('timezone').options[i].value == convert(std_time_offset)+","+dst) {
		    document.getElementById('timezone').selectedIndex = i;
		    break;
	    }
   }
}

function convert(value) {
    var hours = parseInt(value);
    value -= parseInt(value);
    value *= 60;
    var mins = parseInt(value);
    value -= parseInt(value);
    value *= 60;
    var secs = parseInt(value);
    var display_hours = hours;
    display_hours = (hours < 10 && hours > 0) ? "+0"+hours : "+"+hours; // positive
    display_hours = (hours == 0) ? "0"+hours : display_hours; // handle GMT case (00:00)
    display_hours = (hours < 0 && hours > -10) ? "-0"+Math.abs(hours) : display_hours; // neg
    mins = (mins < 10) ? "0"+mins : mins;
    return display_hours+":"+mins;
}

jQuery(document).ready(calculate_time_zone);