РНР промыслового неустранимая ошибка

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у меня есть эта ошибка при вставке $diff в мою таблицу базы данных

time_in(time)

но у меня есть эта ошибка

Catchable fatal error: Object of class DateInterval could not be converted to string

ошибка в моем запросе

Что я уже пробовал:

$conn = mysqli_connect(server,username,password)
        or die (mysqli_error($conn));
mysqli_select_db($conn,database)
            or die (mysqli_error($conn));

if ($now->format("H:i") > "22:00") {
    $deadline = DateTime::createFromFormat("H:i", "22:00");
    $diff = $now->diff($deadline);
    echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
} else if ($now->format("H:i") > "19:00") {
    $deadline = DateTime::createFromFormat("H:i", "19:00");
    $diff = $now->diff($deadline);
    echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
} else if ($now->format("H:i") > "16:00") {
    $deadline = DateTime::createFromFormat("H:i", "16:00");
    $diff = $now->diff($deadline);
    echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
} else if ($now->format("H:i") > "13:00") {
    $deadline = DateTime::createFromFormat("H:i", "13:00");
    $diff = $now->diff($deadline);
    echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
} else if ($now->format("H:i") > "10:00") {
    $deadline = DateTime::createFromFormat("H:i", "10:00");
    $diff = $now->diff($deadline);
    echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
} else if ($now->format("H:i") > "07:00") {
    $deadline = DateTime::createFromFormat("H:i", "07:00");
    $diff = $now->diff($deadline);
    echo "You are ".$diff->h." hours and ".$diff->i." minutes late";
}

$query = "INSERT INTO `time_in`(`late`,`date_in`)
                VALUES ('$diff',NOW())";
                if (!$query) {
                printf("Error: %s\n", mysqli_error($conn));
                exit();
                } 
        if($query = $conn ->query($query)){
            echo "<br>successful";
        } else {
            echo "not successful";
        }

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