Description:
------------
If you diff two dates with TLA timezone (like EDT) and then add the interval back 
to the earliest date, you would not get the later date but instead the result will 
be one hour off. See example below.

Test script:
---------------
$start = new DateTime('2010-06-04 02:18:48 EDT'); 
var_dump($start->format('U'));
$end   = new DateTime('2010-06-06 18:38:28 EDT');  
$ts = $end->format('U');
var_dump($start, $end);
$int = $start->diff($end);
var_dump($int, $start);
var_dump($start->format('U'));
$res = $start->add($int);
var_dump($res);


Expected result:
----------------
.....

object(DateTime)#1 (3) {
  ["date"]=>
  string(19) "2010-06-06 18:38:28"
  ["timezone_type"]=>
  int(2)
  ["timezone"]=>
  string(3) "EDT"
}


Actual result:
--------------
.......

object(DateTime)#1 (3) {
  ["date"]=>
  string(19) "2010-06-06 17:38:28"
  ["timezone_type"]=>
  int(2)
  ["timezone"]=>
  string(3) "EDT"
}