you need to set it in the [preprocess][1] function of the template. or see this answer https://stackoverflow.com/questions/2383865/how-do-i-use-theme-preprocessor-functions-for-my-own-templates

you'll need to hook this preprocess function

    /**
     * Preprocesses template variables for the author pane.
     */
    function advanced_forum_preprocess_author_pane(&$variables) {
      // Author pane is used in various places. Check the caller to make sure
      // we are the one that called it.
      if (!empty($variables['caller']) && $variables['caller'] == 'advanced_forum') {
        advanced_forum_add_template_suggestions("author_pane", $variables);
      }
    }

maybe the [author pane][2] module could do the job for you ;)
here's a similar question https://drupal.stackexchange.com/questions/114908/how-to-modify-author-pane-date-fields

  [1]: https://api.drupal.org/api/drupal/includes%21theme.inc/function/template_preprocess/7
  [2]: https://www.drupal.org/project/author_pane