我通过网上找到了的方法试了一遍,代码是这样的
<script type="text/javascript">
function changeDay(){
var year=parseInt(document.getElementById(bdyear).value);
var month=parseInt(document.getElementById(bdmonth).value);
document.getElementById(bdday).length=0;
int d;
for(d=1;d<=31;d++){
document.getElementById(bdday).options.add(new Option(d, d));
if(((month==4)||(month==6)||(month==9)||(month==11))&&(d==30)){
break;
}else if((year%4!=0)&&(month==2)&&(d==28)){
break;
}else if((year%4==0)&&(month==2)&&(d==29)){
break;
}
}
}
</script>
以及
<table width = "100%" border = "0" cellspacing = "0">
<tr>
<td>
<select id="bdyear" name="bdyear" onchange = "changeDay()" style="width:100%; height:100%;">
<%int y;
for(y=1960;y<=2020;y++){
%>
<option value ="<%=y%>"><%=y%></option>
<%} %>
</select>
</td>
<td>年</td>
<td>
<select id="bdmonth" name="bdmonth" onchange = "changeDay()" style="width:100%; height:100%;">
<%int m;
for(m=1;m<=12;m++){
%>
<option value ="<%=m%>"><%=m%></option>
<%} %>
</select>
</td>
<td>月</td>
<td>
<select id="bdday" name = "bdday" style="width:100%; height:100%;"></select>
</td>
<td>日</td>
</tr>
</table>
就是个根据年月来判断日能选什么,但是写完出来无论怎么选日都选项,小白求指点