DateTime php return a wrong date - php

Question

I'm using the PHP DateTime class to generate a date for a custom licensing system. While I'm debugging it, I've noted that the date time are always wrong, it will be 5-dec-2018 but now we are in November, and this date will be the same also for the expiration_date.
How i can fix this issue? I need to add 30 days to the start date of the trial period.
Here is the code.
class Activator {
private $uuid;
private $keygen;
private $licence_code;
public static function generateLicence($uuid) {
if (!file_exists(ABSPATH.'/DataStorage/.licence')) {
$start_date = new DateTime();
$time_zone = $start_date->setTimezone(new DateTimeZone('Europe/Rome'));
$trial_date = $start_date->add(new DateInterval('P30D'));
$end_date = $trial_date->format('d-M-Y');
$machine_uuid = bin2hex($uuid);
$licence_code = base64_encode($machine_uuid);
$licence_file = array(
'uuid' => $machine_uuid,
'activation_date' => $time_zone->format('d-M-Y'),
#'trial_version' => true,
#'expire_date' => $end_date,
#'licence_code' => $licence_code
);
$w = file_put_contents(ABSPATH.'/DataStorage/.licence', json_encode($licence_file));
echo $w;
}
}

Answer 1

This is the expected behavior, as you add() to the date (by doing $start_date->add(...) - this modifies the original object $start_date.
You can solve this a few different ways, although the easiest way is just to create a new instance entirely with the 30 days added directly in the construct. You can also set the timezone as the second parameter of new DateTime().
$timezone = new DateTimeZone('Europe/Rome');
$start_date = new DateTime("now", $timezone);
$trial_date = new DateTime("+30 days", $timezone);
PHP.net on new DateTime()
PHP.net on DateTime::add()
See this live demo.