Source Reference:

• Component: bbPress :: bbp-topic

  File: /bbp-includes/bbp-topic-functions.php :: Trac Source Line: 2208

  	// Check time and use current if empty
  	if ( empty( $new_time ) )
  		$new_time = get_post_field( 'post_date', bbp_get_public_child_last_id( $topic_id, bbp_get_reply_post_type() ) );
  
  	// Update only if published
  	if ( !empty( $new_time ) )
  		update_post_meta( $topic_id, '_bbp_last_active_time', $new_time );
  
  	return apply_filters( 'bbp_update_topic_last_active_time', $new_time, $topic_id );
  }
  
  /**
   * Update the topic with the most recent reply ID
   *
   * @since bbPress (r2625)
   *
   * @param int $topic_id Optional. Topic id to update
   * @param int $reply_id Optional. Reply id
   * @uses bbp_is_reply() To check if the passed topic id is a reply