Calculating total time taken to finish a race

The following method illustrates how to calculate, time taken by a rider to finish a race.
Terminolgies

• Time taken(field_time_taken): Time taken by a rider to finish the race, which is an arithmetic operation of start time, stop time and penalities.
• Start time(field_start_time): The start time of the race.
• Stop time(field_stop_time): Time at which the rider finishes the race.
• Penalties(field_penalties): Penalties(in seconds).

So Time taken = Stop time - Start time + Penalties

In Drupal 7, $node variable can no longer be used instead $entity has to be used.
The following code, would go into the "Computed Code(PHP)" section of the field settings.

  $node = menu_get_object();
  $start_time = field_get_items('node', $node, 'field_start_time');
  $stop_time = field_get_items('node', $node, 'field_stop_time');
  $start_time = $start_time[0]['value'];
  $stop_time = $stop_time[0]['value'];
  $penalties = field_get_items('node', $node, 'field_penalities');
  $penalties = $penalties[0]['value'];
  $entity_field[0]['value'] = $stop_time - $start_time + $penalties;

The following code would go into "Display Code (PHP)" section of the field settings.

  $display_output = $entity_field_item['value'];