Description:
------------
If you're using a timezone, like Europe/Paris, with summer and winter times, you 
will obtain a wrong result when adding and subtracting a time interval on a date 
at summertime threshold.

I fixed that issue by making all operations on GMT timezone.

Test script:
---------------
date_default_timezone_set('Europe/Paris');

/*
   Get a time just on the threshold
*/

$d = new DateTime("2012-03-25 03:00:00");
echo $d->format(DateTime::RFC2822) . "\n";

/*
    Substract one hour
*/

$i = new DateInterval("PT1H");
$d->sub($i);
echo $d->format(DateTime::RFC2822) . "\n";

Expected result:
----------------
Sun, 25 Mar 2012 03:00:00 +0200
Sun, 25 Mar 2012 01:00:00 +0100

Actual result:
--------------
Sun, 25 Mar 2012 03:00:00 +0200
Sun, 25 Mar 2012 03:00:00 +0200