public function add_single(Hour $hour) {
    $hours = count($this->hours);
    if($hours && $hour->getHour() < $this->hours[$hours-1]->getHour()) {
      $this->past_midnight = True;
    }

    if($this->past_midnight) {
      $hour->pastMidnight();
    }

    $this->hours[] = $hour;
  }