Why does the date of 1970 appear in the code of my page?

<?php 

if(isset($_POST['search'])) { 
$rankselect = $_POST['rankselect'];
$i = 1;
if($rankselect == 5) {
    ?>

        <div class="title-page" style="margin-bottom:15px;">BIG BOSSES</div>
    <table width="691" cellpadding="10" cellspacing="10" border="0">
  <tbody> 
    <tr>
      <td>Name</td>
        <td>Level</td>
      <td>Status</td>
    </tr>
    <?php
    try { 
$queryselectbig = "SELECT b.*,n.id, n.name,n.level FROM grandboss_data b LEFT JOIN npc n ON b.boss_id = n.id  ORDER BY n.name ASC"; 

$querybossesbig = $db -> prepare($queryselectbig); 
$querybossesbig -> execute(); 
while ($resbossesbig = $querybossesbig -> fetch (PDO::FETCH_OBJ)) {
    $respawnbig = $resbossesbig->respawn_time; 
    $convertdatebig = date('d/m/Y H:i:s',($respawn->respawn_time / 1000));
    ?>
       <tr>
      <td><?php echo $resbossesbig-> name; ?></td>
      <td><?php echo $resbossesbig-> level; ?></td>
      <td>
      <?php if($resbossesbig->respawn_time == 0){
echo "<div style='color:green' class='glyphicon glyphicon-ok'></div>";
} else { echo $convertdatebig; }
 ?>
       </td>
    </tr>
    <?php

}
} catch (PDOException $e) {
    echo $e->getMessage();
}
?>
  </tbody>
</table>


    <div class="title-page" style="margin-bottom:15px; clear:both; margin-top:15px;">RAID BOSSES</div>
    <table width="691" cellpadding="10" cellspacing="10" border="0">
  <tbody> 
    <tr>
      <td>Name</td>
        <td>Level</td>
      <td>Status</td>
    </tr>
    <?php
$queryselectbosses = SELECT b.*,n.id, n.name,n.level FROM raidboss_spawnlist b LEFT JOIN npc n ON b.boss_id = n.id ORDER BY n.name ASC ; 

$querybosses = $db -> prepare($queryselectbosses); 
$querybosses -> execute(); 
while ($resbosses = $querybosses -> fetch (PDO::FETCH_OBJ)) {
    $respawn = $resbosses->respawn_time; 
    $convertdate = date('d/m/Y H:i:s',($respawn->respawn_time / 1000));
    ?>
       <tr>
      <td><?php echo $resbosses-> name; ?></td>
      <td><?php echo $resbosses-> level; ?></td>
      <td>
      <?php if($resbosses->respawn_time == 0){
echo "<div style='color:green' class='glyphicon glyphicon-ok'></div>";
} else { echo $convertdate; }
 ?>
       </td>
    </tr>
    <?php
}
}
}

?>

  </tbody>
</table>