Drupal 7: Define Age formatter for date fields.

kalabro.module

<?php
/**
 * Define Age formatter for date fields.
 */

/**
 * Implements hook_field_formatter_info().
 */
function kalabro_field_formatter_info() {
  $formatters = array(
    'date_age' => array(
      'label' => t('Age'),
      'field types' => array('date', 'datestamp', 'datetime'),
      'settings' => array(
        'show_raw_date' => 0,
        'raw_date_format' => 'short',
      ),
    ),
  );

  return $formatters;
}

/**
 * Implements hook_field_formatter_settings_form().
 */
function kalabro_field_formatter_settings_form($field, $instance, $view_mode, $form, &$form_state) {
  $display = $instance['display'][$view_mode];
  $settings = $display['settings'];
  $id = drupal_html_id('show_raw_date');
  $element['show_raw_date'] = array(
    '#type' => 'checkbox',
    '#title' => t('Show full date after age'),
    '#default_value' => $settings['show_raw_date'],
    '#attributes' => array('id' => $id),
  );
  $element['raw_date_format'] = array(
    '#type' => 'select',
    '#title' => t('Date format'),
    '#default_value' => $settings['raw_date_format'],
    '#options' => date_format_type_options(),
    '#states' => array(
      'visible' => array(
       '#' . $id => array('checked' => TRUE),
      ),
    ),
  );
  return $element;
}

/**
 * Implements hook_field_formatter_settings_summary().
 */
function kalabro_field_formatter_settings_summary($field, $instance, $view_mode) {
  $display = $instance['display'][$view_mode];
  $settings = $display['settings'];

  $value = $settings['show_raw_date'] ? t('yes') : t('no');
  $summary = t('Show full date after age') . ': <em>' . $value . '</em>';

  return $summary;
}

/**
 * Implements hook_field_formatter_view().
 */
function kalabro_field_formatter_view($entity_type, $entity, $field, $instance, $langcode, $items, $display) {
  $element = array();
  $settings = $display['settings'];

  if ($display['type'] == 'date_age') {
    foreach ($items as $delta => $item) {
      if (!empty($item['value'])) {
        $age = _ij_main_count_age($item['value']);
        $element[$delta] = array('#markup' => $age);
        if ($settings['show_raw_date']) {
          $element[$delta]['#suffix'] = ' (' . format_date(strtotime($item['value']), $settings['raw_date_format']) . ')';
        }
      }
    }
  }

  return $element;
}

/**
 * Helper for age count.
 */
function _kalabro_count_age($date_str) {
  $diff = date_diff(new DateTime($date_str), new DateTime());
  return $diff->y;
}

What is _ij_main_count_age()? Should it be _kalabro_count_age() instead?