Piwik\Date::subMonth PHP Method

    public function subMonth($n)
    {
        if ($n === 0) {
            return clone $this;
        }
        $ts = $this->timestamp;
        $result = mktime(date('H', $ts), date('i', $ts), date('s', $ts), date('n', $ts) - $n, 1, date('Y', $ts));
        return new Date($result, $this->timezone);
    }