function prepareYears() {
	var yearList = document.getElementById("yearlist");
	var thisYear = new Date();
	thisYear = thisYear.getFullYear()-3;
	for (var i = 0; i < 6; i++) {
		yearList.options[i] = new Option(thisYear, thisYear);
		thisYear++;
	}
	yearList.options.selectedIndex = 3;
}

window.onload = function() {
	prepareYears();

}

function fixDays(form) {
var month = form.month.value;
var year = form.year.value;
if(month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) {
	form.day.options[30] = new Option("31", "31");
	form.day.options[29] = new Option("30", "30");
	form.day.options[28] = new Option("29", "29");
} else if (month == 4 || month == 6 || month == 9 || month == 11) {
	form.day.options[30] = null;
	form.day.options[29] = new Option("30", "30");
	form.day.options[28] = new Option("29", "29");
} else if (month == 2 && (year % 4 == 0) && ((year % 100 != 0) || ((year % 100 == 0) && (year % 400 == 0)))) {
	form.day.options[30] = null;
	form.day.options[29] = null;
	form.day.options[28] = new Option("29", "29");
} else {
	form.day.options[30] = null;
	form.day.options[29] = null;
	form.day.options[28] = null;
}
}

function dueDate(form) {
var exp = document.getElementById('exp');
var duedate = document.getElementById('duedate');
var monthNames = new Array("January", "February", "March", "April", "June", "July", "August", "September", "October", "November", "December");
var dayNames = new Array("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday");
var menstrual = document.getElementById("menstruation").checked;
var conception = document.getElementById("conception").checked;
var month = form.month.value-1;
var day = form.day.value;
var year = form.year.value;
var date = day+" "+month+" "+year;
var startDate = new Date();
startDate.setDate(day);
startDate.setMonth(month);
startDate.setFullYear(year);
if (menstrual) {
	var addition = 280;
} else if (conception) {
	var addition = 266;
}
startDate.setTime(startDate.valueOf()+(addition*86400000));
var newday = startDate.getDay();
var newdate = startDate.getDate();
var newmonth = startDate.getMonth();
var newyear = startDate.getFullYear();
var datestring = dayNames[newday]+", "+monthNames[newmonth-1]+" "+newdate+", "+newyear;
exp.firstChild.nodeValue = "Your due date is ";
duedate.firstChild.nodeValue = datestring;
exp.lastChild.nodeValue = ".";
return false; 
}