Description:
------------
When you use a timezone with daylight saving time (like Europe/Berlin) and the date of the system running the PHP script is in this daylight saving time (like 2012-06-15), DateTime::add() will produce results that are off by 1 hour.

When the date on the local machine is not in the daylight saving time, the same computation will yield the expceted results.


This bug also only appears, when the DateTime instance was created using a timestamp, so a workaround would be the following: Instead of

  $date = new DateTime(gmmktime(0, 0, 0, 1, 1, 2012));

use

  $date = new DateTime('2012-01-01');

The script was tested with the following systems, all having the same problem:
- Windows XP, PHP 5.3.10
- Windows XP, PHP 5.3.13
- Windows 7, PHP 5.3.10
- Arch Linux (x86_64), PHP 5.4.3
- Debian Squeeze, PHP 5.3.10

Test script:
---------------
<?php
$oneMonth = new DateInterval('P1M');

$date = new DateTime('@'.gmmktime(0, 0, 0, 1, 1, 2012));
echo $date->format('Y-m-d H:i:s')."\n";

$date->add($oneMonth);
echo $date->format('Y-m-d H:i:s')."\n";

Expected result:
----------------
2012-01-01 00:00:00
2012-02-01 00:00:00

Actual result:
--------------
2012-01-01 00:00:00
2012-02-01 01:00:00