date selector

<?php
/********* DATE SELECTOR ************
//  created by  : Cahya DSN
//  create date : 2012-10-03
*************************************/
// default selisih tgl berangkat dgn hari pulang
$selisih_hari=3;
?>
<!DOCTYPE html>
<html>
<head>
  <title>Date Selector</title>
  <style type="text/css">
    * {margin:0;padding:0;}
    body {font-family:verdana,sanf-serif;color:#009;font-size:10px;}
    select {background-color:#eee;border:solid 1px #999;font-family:verdana,sanf-serif;color:#009;font-size:10px;}
    .container {width:150px;padding:5px;margin:5px;}
    fieldset {padding:5px;margin:5px;}
  </style>
</head>
<body>
<div class="container">
<fieldset>
<legend>Tanggal Berangkat</legend>
<select name="day" id="day" size="1">
<?php
  for($d=1;$d<=date("t");$d++){
    echo "<option value=\"$d\"".(date("d")==$d?" selected=\"selected\"":"").">".$d."</option>\n";
  }
?>
</select>
<select name="yearmonth" id="yearmonth" onChange="setDay('yearmonth','day');">
<?php
for($i=0;$i<12;$i++){
  $nextmonth = mktime(0, 0, 0, date("m")+$i, date("d"),   date("Y"));
  echo "<option value=\"".date("Ym",$nextmonth)."\">".date("M Y",$nextmonth)."</option>";
}
?>
</select>
</fieldset>
<fieldset>
<legend>Tanggal Pulang</legend>
<select name="day1" id="day1" size="1">
<?php
  for($d=1;$d<=date("t");$d++){
    echo "<option value=\"$d\"".(date("d",mktime(0,0,0,date("m"),date("d")+$selisih_hari,date("Y")))==$d?" selected=\"selected\"":"").">".$d."</option>\n";
  }
?>
</select>
<select name="yearmonth1" id="yearmonth1" onChange="setDay('yearmonth1','day1');">
<?php
for($i=0;$i<12;$i++){
  $nextmonth = mktime(0, 0, 0, date("m")+$i, date("d"),   date("Y"));
  echo "<option value=\"".date("Ym",$nextmonth)."\">".date("M Y",$nextmonth)."</option>";
}
?>
</select>
</fieldset>
</div>
<script>
  function setDay(id_ym,id_d) {
    var ym = document.getElementById(id_ym);
    var dy = document.getElementById(id_d);
    var dym = ym.options[ym.selectedIndex].value;
    var ddy = dy.options[dy.selectedIndex].value;
    var year=parseInt((dym.toString()).substr(0,4));
    var month=parseInt((dym.toString()).substr(4,2));
    var dd = new Date(year, month, 0);
    var days = dd.getDate();
    dy.options.length = 0;
    for (var i = 1; i <= days; i++)
      dy.options[dy.options.length] = new Option(i,i);

  }
</script>
</body>
</html>