function daysInMonth(month,year) {
var dd = new Date(year, month, 0);
return dd.getDate();
} 
function setDayDrop(dyear, dmonth, dday) {
var year = dyear.options[dyear.selectedIndex].value;
var month = dmonth.options[dmonth.selectedIndex].value;
var day = dday.options[dday.selectedIndex].value;
if (day == ' ') {
var days = (year == ' ' || month == ' ') ? 31 : daysInMonth(month,year);
dday.options.length = 0;
dday.options[dday.options.length] = new Option(' ',' ');
for (var i = 1; i <= days; i++) 
dday.options[dday.options.length] = new Option(i,i);
}}

function setDay() {
var year = document.getElementById('year');
var month = document.getElementById('month');
var day = document.getElementById('day');
setDayDrop(year,month,day);
}
document.getElementById('year').onchange = setDay;
document.getElementById('month').onchange = setDay;