Description:
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Incorrect data is generated by function DateTime::__construct in case of using the 
relative definition of time.

If you use the command "last day of month" the previous month will be returned.
If you use the command "first day of month" the correct month will be returned and 
second day in the month will be returned.

Test script:
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$datea = new DateTime('last day October 2011');
echo $datea->format('Y-F-d');

$dateb = new DateTime('last monday October 2011');
echo $dateb->format('Y-F-d');

$datec = new DateTime('first day October 2011');
echo $datec->format('Y-F-d');

$datec = new DateTime('first day November 2011');
echo $datec->format('Y-F-d');


Expected result:
----------------
2011-October-30
2011-October-30
2011-October-01
2011-November-01


Actual result:
--------------
2011-September-30
2011-September-26
2011-October-02
2011-November-02