三级轮播

<!DOCTYPE html>
<html>
   <head>
       <meta charset="UTF-8">
       <title></title>
   </head>
   <body>
       <select size="1" id="a1" style=" 60px;"></select>
       <select size="1" id="a2" style=" 50px;"></select>
       <select size="1" id="a3" style=" 50px;"></select>
   <body>
</html>
<script>
   a1=document.getElementById('a1')
   a2=document.getElementById('a2')
   a3=document.getElementById("a3")
   for(i=1900;i<2018;i++){
   o=document.createElement('option')
   o.innerText=i
   a1.appendChild(o)
   }
   for(j=1;j<13;j++){
   p=document.createElement('option')
   p.innerText=j
   a2.appendChild(p)
   }
   for(k=1;k<32;k++){
       q=document.createElement('option')
       q.innerText=k
       a3.appendChild(q)
   }
   a2.onchange=function(){
       a3.options[28].style.display="inline"
       a3.options[29].style.display="inline"
       a3.options[30].style.display="inline"
       n=a1.selectedOptions[0].value
       m=a2.selectedOptions[0].value
       if(n%4==0&&n%100!=0||n%400==0){
           if(m==2){
               a3.options[30].style.display="none"
               a3.options[29].style.display="none"
                }else{
                   if(m==4||m==6||m==9||m==11){
                       a3.options[30].style.display="none"
                   }
                }
           }else{
               if(m==2){
                   a3.options[28].style.display="none"
                   a3.options[30].style.display="none"
                    a3.options[29].style.display="none"
               }else{
                   if(m==4||m==6||m==9||m==11){
                       a3.options[30].style.display="none"
                   }
               }
           }
       }
   a1.onchange=function(){
       a3.options[28].style.display="inline"
       a3.options[29].style.display="inline"
       a3.options[30].style.display="inline"
       n=a1.selectedOptions[0].value
       m=a2.selectedOptions[0].value
       if(n%4==0&&n%100!=0||n%400==0){
           if(m==2){
               a3.options[30].style.display="none"
               a3.options[29].style.display="none"
                }else{
                   if(m==4||m==6||m==9||m==11){
                       a3.options[30].style.display="none"
                   }
                }
           }else{
               if(m==2){
                   a3.options[28].style.display="none"
                   a3.options[30].style.display="none"
                    a3.options[29].style.display="none"
               }else{
                   if(m==4||m==6||m==9||m==11){
                       a3.options[30].style.display="none"
                   }
               }
           }
       }