public static function rangeLastMonth()
    {
        $lastMonth = date('m') - 1;
        if ($lastMonth <= 0) {
            $lastMonth = 12;
        }
        $lastMonth = date('Y') . '-' . $lastMonth . '-01';

        return [self::toTs($lastMonth), self::toTs($lastMonth) + (86400 * (date('t', strtotime($lastMonth))-1))];
    }