cara rubah Waktu Ujian

Pak Roby saya coba bikin tombol edit batas waktu, ada sedikit masalah di penyimpanan mohon di cek apakah ada yang salah di script update nya.
terima kasih.
 
ini scriptnya 
 
}elseif($_GET[act]=='tambah'){
cek_session_admin();
if (isset($_POST[tambah])){
if(function_exists('date_default_timezone_set')) date_default_timezone_set('Asia/Jakarta');
$waktu = date("Y-m-d H:i:s");
$date = date_create($waktu);
$tjam = date_add($date, date_interval_create_from_date_string("$_POST[c] minutes"));
$bataswaktu = date_format($tjam, 'Y-m-d H:i:s');
mysql_query("INSERT INTO rb_quiz_ujian VALUES ('','$_POST[a]','$_GET[jdwl]','$_POST[b]','$bataswaktu')");
echo "[removed][removed]='index.php?view=soal&act=listsoal&jdwl=".$_GET[jdwl]."&id=".$_GET[id]."&kd=".$_GET[kd]."';[removed]";
}
echo "<div class='col-md-12'>
<div class='box box-info'>
<div class='box-header with-border'>
<h3 class='box-title'>Tambah List Ujian dan Quiz Baru</h3>
</div>
<div class='box-body'>
<form method='POST' class='form-horizontal' action='' enctype='multipart/form-data'>
<div class='col-md-12'>
<table class='table table-condensed table-bordered'>
<tbody>
<tr><th width='120px' scope='row'>Kategori</th> <td><select class='form-control' name='a'> 
<option value='0' selected>- Pilih Kategori -</option>"; 
$kategori = mysql_query("SELECT * FROM rb_kategori_quiz_ujian");
while($a = mysql_fetch_array($kategori)){
echo "<option value='$a[id_kategori_quiz_ujian]'>$a[kategori_quiz_ujian]</option>";
}
echo "</select>
</td></tr>
<tr><th scope='row'>Keterangan</th> <td><input type='text' class='form-control' name='b'></td></tr>
<tr><th scope='row'>Batas Waktu</th> <td><input [removed]='width:20%' type='text' class='pull-left form-control' name='c'> Menit</td></tr>
</tbody>
</table>
</div>
</div>
<div class='box-footer'>
<button type='submit' name='updatebw' class='btn btn-info'>Update</button>
<a href='index.php?view=bahantugas'><button class='btn btn-default pull-right'>Cancel</button></a>
</div>
</form>
</div>";