一个多个关联的下拉列表框怎么做？

<html>
<head>
<script language="JavaScript">
<!--
var objReport=null;
var objYear=null;
var objMonth=null;
var objDate=null;
function init(){
objReport=document.getElementById("dltReport");
objYear=document.getElementById("dltYear");
objMonth=document.getElementById("dltMonth");
objDate=document.getElementById("dltDate");
objReport.attachEvent("onchange",RptChg);
objYear.attachEvent("onchange",YearChg);
objMonth.attachEvent("onchange",MonthChg);
var date = new Date();
var curYear=date.getYear();
var curMonth=date.getMonth()+1;
var curDate=date.getDate();
if(objYear == null || objMonth == null || objDate == null ){
alert("Can't not find object!"); 
return false;
}
var str="";
optAdd(objReport,"年报");
optAdd(objReport,"月报");
optAdd(objReport,"日报");
initDltValue(objYear,1990,2050);
initDltValue(objMonth,1,12);
initDltValue(objDate,1,CalDays(curYear,curMonth));
objReport.value="日报";
objYear.value=curYear;
objMonth.value=curMonth;
objDate.value=curDate;} 
function initDltValue(obj,min,max){
for(var i=min;i<=max;i++)
optAdd(obj,i);
}
function RptChg(){
 setDisabled(objReport.options[objReport.selectedIndex].value);
}
function YearChg(){
var dateVal=objDate.value;
objDate.innerHTML="";
initDltValue(objDate,1,CalDays(parseInt(objYear.value),parseInt(objMonth.value)));
objDate.value=dateVal;
if(objDate.selectedIndex==-1) objDate.selectedIndex=0;
}
function MonthChg(){
objDate.innerHTML="";
initDltValue(objDate,1,CalDays(parseInt(objYear.value),parseInt(objMonth.value)));
}
function setDisabled(val){
switch(val){
case '年报':
objYear.disabled=false;
objMonth.disabled=true;
objDate.disabled=true;
break;
case '月报':
objYear.disabled=false;
objMonth.disabled=false;
objDate.disabled=true;
break;
case '日报':
objYear.disabled=false;
objMonth.disabled=false;
objDate.disabled=false;
break;
}
}
function optAdd(obj,val){
var newOpt=document.createElement("option");
obj.add(newOpt);
newOpt.value=val;
newOpt.text=val;
}
function CalDays(year,mon){
switch(mon){
case 1: case 3: case 5: case 7: case 8: case 10: case 12: return 31;
case 4: case 6: case 9: case 11: return 30;
case 2: if((year%100!=0 && year%4 == 0) || year % 400 == 0) return 29; else return 28;
}
}
//-->
</script>
</head>
<body onLoad="init()">
<table cellpadding=0 cellspacing=0 border=0 width=400>
<tr>
<td>
<select id="dltReport" style="width:100%"></select>
</td>
<td>
<select id="dltYear" style="width:100%"></select>
</td>
<td>
<select id="dltMonth" style="width:100%"></select>
</td>
<td>
<select id="dltDate" style="width:100%"></select>
</td>
</tr>
</table>
</body>
</html>