- var_dump: documentation ( source)
- date_default_timezone_set: documentation ( source)
<?php
date_default_timezone_set('Europe/Berlin');
$date = new DateTime('2000-12-31');
$date->add(new DateInterval('P1M'));
var_dump($date->format('Y-m-d'));
$date->add(new DateInterval('P1M'));
var_dump($date->format('Y-m-d'));