ทำไมใช้ AND ใน script ของ checkform ไม่ได้ครับผม

<form name="setup" method="post" action="" ENCTYPE = "multipart/form-data" onsubmit="return checkform(this);">
    <input type="text" id="month" name="month">
    <input type="checkbox" id="unlimited" name="unlimited" value="Unlimited"/> Unlimited 
    <input type="submit" name="submit" value="Next" />
</form>

<script language="JavaScript" type="text/javascript">
<!--
function checkform ( form )
{
  if ((form.month.value == "" ) AND (form.unlimited.checked == false )) 
  {
    alert( "Please enter Month." );
    form.month.focus();
    return false ;
  }
return true ;
}
//-->
</script>