Change date format in Advanced Forum
You need to alter the date through template_preprocess_node() and template_preprocess_comment() in your theme. The first preprocess is used for the forum nodes (initial forum post) and the latter for comments (replies).

Add the following code to your theme's template.php file:

function YOUR_THEME_preprocess_node(&amp;$variables) {
  if ($variables['node']-&gt;type=='forum') {
    // Custom date.
    $variables['date'] = format_date($variables['node']-&gt;created, 'custom', 'M d Y, H:i:s');
    // Use date format provided by Drupal:
    // $variables['date'] = format_date($variables['node']-&gt;created, 'short');
  }
}

function YOUR_THEME_preprocess_comment(&amp;$variables) {
  if ($variables['node']-&gt;type=='forum') {
    // Custom date.
    $variables['date'] = format_date($variables['node']-&gt;created, 'custom', 'M d Y, H:i:s');
    // Use date format provided by Drupal:
    // $variables['date'] = format_date($variables['node']-&gt;created, 'short');
  }
}