Description:
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British summertime does not actually end until....
27-10-2013 01:59:59+01:00  (BST)
at which point, one second later it kicks back to GMT...
27-10-2013 01:00:00+00:00 (GMT)

It seems calculations involving DateTime->add() and the 'Europe/London' DateTimeZone crossing the END of British summertime (BST) gives an incorrect result. 

On the day British summertime ends it is not until 2:00AM (BST) which is 1:00AM (GMT) that clocks switch from BST to GMT (i.e. go back from 2:00 to 1:00) 

Test script:
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<?php
$start = new DateTime("2013-10-27 00:55:00", new DateTimeZone('Europe/London'));
$step = DateInterval::createFromDateString("600 seconds");
echo "start = " . $start->format('d-m-Y H:i:sP') . "<br />";
$start->add($step);
echo "start = " . $start->format('d-m-Y H:i:sP') . "<br />";
?>


Expected result:
----------------
start = 27-10-2013 00:55:00+01:00
start = 27-10-2013 01:05:00+00:00


Actual result:
--------------
start = 27-10-2013 00:55:00+01:00
start = 27-10-2013 01:05:00+01:00