อยากสอบถามวิธีการทำ show/hide <div> กับ <select> list ครับ

<input name="login" id="login" class="login" type="button" onClick="if(document.getElementById('signin') .style.display=='none') {document.getElementById('signin') .style.display=''}else{document.getElementById('signin') .style.display='none'}" title="<?php echo $lan['sign_in'];?>">

<div id="signin">...</div>

<script language="javascript">
for(i=1;i<=12;i++){
	if(i<10){
		var j = '0'+i;
		}else{
			var j = i;
			}
	
	}
	var m = document.getElementById("mm").value;
	switch(m){
		case '01'||'03'||'05'||'07'||'08'||'10'||'12': var d = 31;
		case '04'||'06'||'09'||'11':var d = 30;
		case '02':var d = 28;
				}
	for(t=1;t<=d;t++){
		if(t<10){
		var dd = '0'+t;
		}else{
			var dd = t;
			}
		}
</script>

<select name="mm" class="mm" id="mm">
 <option><?php echo $lan['month'];?></option>
 <option value="Javascript:j"><script language="javascript">document.write("j");</script></option>
  </select>

<select name="dd" class="dd" id="dd">
  <option><?php echo $lan['date'];?></option>
 <option value="Javascript:dd"><script language="javascript">document.write("dd");</script></option>
 </select>