private static function updateTimezone(Timezone $tz)
 {
     $query = "UPDATE timezone SET name = :name, city = :city, utc_offset = :utc_offset\n                  WHERE id = :id";
     $params = array("id" => $tz->getId(), "name" => $tz->getName(), "city" => $tz->getCity(), "utc_offset" => $tz->getUtcOffset());
     $stmt = static::db()->prepare($query);
     $stmt->execute($params);
 }