// script by Josh Fraser (http://www.onlineaspect.com)

function calculate_time_zone() {
	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;
	// check just to avoid error messages
	if (document.getElementById('timezone')) {
		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;
	// handle GMT case (00:00)
	if (hours == 0) {
		display_hours = "00";
	} else if (hours > 0) {
		// add a plus sign and perhaps an extra 0
		display_hours = (hours < 10) ? "+0"+hours : "+"+hours;
	} else {
		// add an extra 0 if needed 
		display_hours = (hours > -10) ? "-0"+Math.abs(hours) : hours;
	}
	
	mins = (mins < 10) ? "0"+mins : mins;
	return display_hours+":"+mins;
}

onload = calculate_time_zone;