JavaScript实现生日选择效果

<script type="text/javascript">
     function pageInit(){
         makeYear();
         makeMonth();
         makeDay();
         document.getElementById("year").onchange = makeDay;
         document.getElementById("month").onchange = makeDay;
     }     
     function makeYear(){
   //填充年份
   var year = document.getElementById("year");
   //如果year是""(空串)删除
   if(year.value=="")
    year.options.length=0;
   //添加option
   //今年是何年
   var now = new Date().getFullYear();
   for(var x=1990; x<=now; x++){
    /*
    var option = document.createElement("option");
    option.value = x;
    option.innerHTML = x;//option.innerText = x;
    year.appendChild(option);
    */
    if(year.value==x)
     continue;
    year.options.add(new Option(x,x));
   }
        }  
     function makeMonth(){         
   //填充月份
   var month = document.getElementById("month");
   //如果month是""(空串)删除
   if(month.value=="")
    month.options.length=0;
   for(var x=1; x<=12; x++){
    if(month.value==x)
     continue;
    month.options.add(new Option(x,x));
   }
        }
        function makeDay(){
   //填充日期
   var day = document.getElementById("day");
   //如果day是""(空串)删除
   if(day.value=="")
    day.options.length=0;
   else
    day.options.length=1;
   var daysNum = getDaysNum();
   for(var x=1; x<=daysNum; x++){
    if(day.value==x)
     continue;
    day.options.add(new Option(x,x));
   }
        }     
        function getDaysNum(year,month){
            //获取天数
         var year = document.getElementById("year").value;
         var month = document.getElementById("month").value;
         var temp = new Date(year,month,0);
         var getDays = new Date(temp.getTime()-1000*60*60*24);
         return temp.getDate();   
        }       
        window.onload = pageInit;

</script>