date_field_modify_migrate.php

<?php

/**
 * Migrate CCK date start with optional duration to date start and date end.
 */
function _apci_aldt_migrate_group_dep_migrate_event_date(&$source_node, &$target_node) {

  $target_node->field_date = array();
  $duration = $source_node->field_duration[0]['value'];

  foreach ($source_node->field_start_date as $source_date) {

    // Convert duration [(string) minutes] to end date.
    if ($duration) {
      $end_date = date_make_date(
        $source_date['value']
      );

      date_modify($end_date, '+'. $duration . ' minutes');

      $source_date['value2'] = date_format(
        $end_date,
        date_type_format(DATE_ISO)  //This probably shouldn't be hard coded.
      );
    }

    // Else, set end time = start time.
    else {
      $source_date['value2'] = $source_date['value'];
    }
    
    $target_node->field_date[] = $source_date;
  }
}

?>