selenium - How to subtract two system times using Java

I have written a Selenium script and I want to calculate the total execution time of the script. How do I subtract the time returned by system when the script started executing and when it’s ended?

I'm using Date and SimpleDateFormat class of Java to get the system time and then format it but the method seems wrong that I'm following to return total time.

    String dateStart = "01/14/2012 23:05:49";
    String dateStop = "01/15/2012 23:07:00";

    SimpleDateFormat sdf = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");

    Date d1 = null;
    Date d2 = null;

    try {

        d1 = sdf.parse(dateStart);
        d2 = sdf.parse(dateStop);

        //in milliseconds
        long diff = d2.getTime() - d1.getTime();

        long diffSeconds = diff / 1000 % 60;
        long diffMinutes = diff / (60 * 1000) % 60;
        long diffHours = diff / (60 * 60 * 1000) % 24;
        //long diffDays = diff / (24 * 60 * 60 * 1000);


        //System.out.print(diffDays + " days, ");
        System.out.print(diffHours + " hours, ");
        System.out.print(diffMinutes + " minutes, ");
        System.out.print(diffSeconds + " seconds.");

    } catch (Exception e) {
        e.printStackTrace();
    }