javascript实现年月日的联动选择

<script type="text/javascript">
   
  function dayChange(year,month,day){
   var selectYear = document.getElementById(year);
   var selectMonth = document.getElementById(month);
   var selectDay = document.getElementById(day);
   var y = selectYear.value,m = selectMonth.value,d = selectDay.value;
   if(m==4||m==6||m==9||m==11){
    if(selectDay.length == 31){
     selectDay.options.remove(30);
    }
    if(selectDay.length == 29){
     selectDay.options.add(30,30);
    }
    if(selectDay.length == 28){
     selectDay.options.add(29,29);
     selectDay.options.add(30,30);
    }
   }
   if(m==1||m==3||m==5||m==7||m==8||m==10||m==12){
    if(selectDay.length == 30){
     selectDay.options.add(new Option(31,31));
    }
    if(selectDay.length == 29){
     selectDay.options.add(new Option(30,30));
     selectDay.options.add(new Option(31,31));
    }
    if(selectDay.length == 28){
     selectDay.options.add(new Option(29,29));
     selectDay.options.add(new Option(30,30));
     selectDay.options.add(new Option(31,31));
    }
   }
   if(m==2){
    if(isRunYear(y)){
     if(selectDay.length == 31){
      selectDay.options.remove(30);
      selectDay.options.remove(29);
     }
     if(selectDay.length == 30){
      selectDay.options.remove(29);
     }
     if(sele