javascript實現年月日的聯動選擇

<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(selectDay.length == 28){
      selectDay.options.add(new Option(29,29));
     }
    }else{
     if(selectDay.length == 31){
      selectDay.options.remove(30);
      selectDay.options.remove(29);
      selectDay.options.remove(28);
     }
     if(selectDay.length == 30){
      selectDay.options.remove(29);
      selectDay.options.remove(28);
     }
     if(selectDay.length == 29){
      selectDay.options.remove(28);
     }
    }
   }      
  }
 

//計算是否為潤年，是返回true，否則返回false
  function isRunYear(tmp_year){
    if(tmp_year%100 == 0){
     if(tmp_year%400 == 0)
      return true;
     else
      return false;
    }else if(tmp_year%4 == 0){
     return true;
    }else{
     return false;
    }
  }
 </script>