看半天不知道错哪了，怎么想都应该是对的呀，就是不知到错哪了，郁闷~

<HTML>
<HEAD>
<title>WebForm2</title>
<meta content="Microsoft Visual Studio .NET 7.1" name="GENERATOR">
<meta content="C#" name="CODE_LANGUAGE">
<meta content="JavaScript" name="vs_defaultClientScript">
<meta content="http://schemas.microsoft.com/intellisense/ie5" name="vs_targetSchema">
<script>
function foption(N){
var i,N,str,M;
str="";
for(i=1;i<=N;i++)
  {if(i<10) str="<option value=''0"+i+"''>"+"0"+i+"</option>";
   else str="<option value=''"+i+"''>"+i+"</option>";   
      document.write(str);
  }  
}function changeri(){
var nian,yue,ri;
ri=31;
yue=form1.selmonth.value;
alert(yue); //为什么总是空
if(yue=="04" || yue=="06" || yue=="09" || yue=="11")
  ri=30;  
if(yue=="02")
  {nian=form1.selyear.value;
  if((nian%4==0 && nian%100!=0) || nian%400==0)
   ri=29;
   else ri=28; 
  }
 flen=form1.selday.length ; 
 form1.selday.length =ri;
 i=flen+1;
 for(i;i<=ri;i++)
   {
    form1.selday.options(i-1).text=i;
    form1.selday.options(i-1).value=i;
   }
}
</script>
</HEAD>
<body MS_POSITIONING="GridLayout">
<form name="form1" method="post"> 
<select name="selyear" onchange="javascript:changeri();">
         <script language="javascript">foption(12);</script>
        </select>年<select id="selmonth" name="selmonth" onchange="javascript:changeri();">
         <script language="javascript">foption(12);</script>
        </select>月<select name=selday>
         <script language="javascript">foption(31);</script>
        </select>日
</form>  </body>
</HTML>