How Make A Simple Timer

<?php
    session_start();
    $_SESSION['test_started'] = date("Y-m-d H:i:s");
    $_SESSION['test_end'] = date("Y-m-d H:i:s",strtotime(date("Y-m-d H:i:s")) + 60*30;);

    echo date("Y-m-d H:i:s",strtotime(date("Y-m-d H:i:s"))-$_SESSION['test_end'];

?>