function formSetDay(){
    var lastday = formSetLastDay($('.selectYear').val(), $('.selectMonth').val());
    var option = '<option value="">--</option>';
    for (var i = 1; i <= lastday; i++) {
      if (i === $('.selectDay').val()){
        option += '<option value="' + i + '" selected="selected">' + i + '</option>\n';
      }else{
        option += '<option value="' + i + '">' + i + '</option>\n';
      }
    }
    $('.selectDay').html(option);
  }

function formSetLastDay(year, month){
    var lastday = new Array('', 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
    if ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0){
      lastday[2] = 29;
    }
    return lastday[month];
}

$('.selectYear, .selectMonth').on('change',function(){
    formSetDay();
});