PHP为什么DateTime diff函数总是返回零？

我有下面的代码试图计算工作时间。我有点问题，小时和分钟总是归零。我最好的猜测是它与新的日期时间有关，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 />";
?>

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这两次之间的时间差为零，所以这是有意义的。您需要使用两个相距较远的时间进行测试，以在此处看到大于零的值

您的会议记录不正确，因为

m

是月份标识符<代码>i用于分钟

$minutes = $diff->i;

$minutes = $diff->i;