使用PHP显示等待时间

我想用这个简单的计算来显示病人的等待时间;

waiting time (the clock time - the arrival time)

在插入表单中;我使用下面的代码将current_time (arrival_time)插入到已经成功的PATIENT表中。

// validate arrival time 
date_default_timezone_set('Europe/London');
$time = time();
$sql="INSERT INTO Patient(Forename, Surname, Gender, Date_Of_Birth, Address, Patient_History, Illness, Priority, Arrival_Time)
VALUES('$patient_name', '$patient_lastname', '$gender', '$date', '$address', '$history', '$illness', '$priority', '$time')";

下面是输出代码;

<?php
$conn = mysqli_connect("localhost","root","") or die ("No connection");
mysqli_select_db($conn, "a&e") or die('Could not select database.');
$query = "SELECT PatientID, Forename, Surname, Gender, Illness, Priority, Arrival_Time, TIMEDIFF(Arrival_time,NOW()) as Waiting_Time FROM Patient";
$result = mysqli_query($conn, $query) or die("Invalid query");
echo "<table border='1'>
<tr>
<th>PatientID</th>
<th>Forename</th>
<th>Surname</th>
<th>Gender</th>
<th>Illness</th>
<th>Priority</th>
<th>Waiting_Time</th>
</tr>";
 while ($row = $result->fetch_object()){

  echo "<tr>
  <td>" . $row->PatientID . "</td>
  <td>" . $row->Forename . "</td>
  <td>" . $row->Surname . "</td>
  <td>" . $row->Gender . "</td>
  <td>" . $row->Illness . "</td>
  <td>" . $row->Priority . "</td>
  <td>" . $row->Waiting_Time . "</td>
  </tr>";
}
echo "</table>";
mysqli_close($conn);
?>

如何使用下面的计算来计算等待时间;(时钟时间-到达时间)

**请注意:Arrival_Time类型为TIME。