PHP为什么DateTimediff函数总是返回零

我有下面的代码，试图计算工作小时数。我有点问题，因为小时和分钟总是归零。我的最佳猜测是这与新的DateTime有关，但我不知道如何补救。

<?php
include_once 'db_connect.php';
include_once 'psl-config.php';
include_once 'functions.php';
$error_msg = "";
sec_session_start();
//Get Username from session id
$username =  htmlentities($_SESSION['username']);
//get time date stamp
date_default_timezone_set('America/Chicago'); //set default timezone, just to be sure

//GET Last Clockin from timesheet table
$result = $mysqli->query("SELECT lastclockin FROM timesheet WHERE username = '$username'");
$number_of_rows =  $result->num_rows;
while ($row = mysqli_fetch_assoc($result)) {
        $lastclockin = $row["lastclockin"];
    }
    echo $lastclockin;
    echo "<br />";
    $now = new DateTime();
    $diff = $now->diff(new DateTime($lastclockin));
$hours = $diff->h;
$minutes = $diff->m;
$hours = $hours + ($diff->days*24);
echo $now->format('Y-m-d H:i:s');
echo "<br />";
echo $hours;
echo "<br />";
echo $minutes;
echo "<br />";
?>

结果如下：

2014-09-11 13:02:15
2014-09-11 13:47:34
0
0