Date format for Comment(time ago?):

You can change the date format for comment using template_preprocess_comment(). For example, normally in comments, the comment created time is displayed like Thu, 12/17/2015 - 23:04. We can change the format to look like 3 weeks 5 Days ago.  

/**
* Implements hook_preprocess_comment
*/
function THEME_NAME_preprocess_comment(&$variables) {
  // Getting the node creation time stamp from the comment object.
  $date = $variables['comment']->getCreatedTime();
  // Here you can use drupal's format_date() function, or some custom php date formatting.
  $variables['created'] = \Drupal::service('date.formatter')->formatInterval(REQUEST_TIME - $date);
  $variables['submitted'] = t('@username commented !datetime', array('@username' => $variables['author'], '!datetime' => '<span class="comments-ago">' . $variables['created'] . ' ago </span>'));
}