determining time difference between two timestamp fields.

Here is the situation. I have a form which sets a timestamp when a user logs in using UPDATE in SQL. The field is called 'login_timestamp' and is in a table called 'Candidates'. I have another timestamp which is set when a user submits the form data into the DB and it is called 'submit_timestamp' . What I want to do is determine the amount of time the user takes to complete the form by subtracting the 'login_timestamp' time form the 'submit_timestamp' time. I am using SQL to extract the data here. $sql = "SELECT Responses.name,Answers,submit_timestamp,login_timestamp FROM Responses LEFT JOIN Candidates USING (user_id)"; and then to display the timestamp in readable form. echo "<tr><th>Completion Time:</th></tr><tr><td>" . date('F j, Y g:i:sa', strtotime($row["login_timestamp"])) . "</td></tr>"; so I need to know how to subtract from two timestamp fields, two different tables and come up with the difference in minutes. thanks. -- ::Bruce::