Description:
------------
DateTime::sub() does not work on objects created by a date string. If the object 
is created by timestamp it works.

Test script:
---------------
<?php                                                                                                                 
$dt = new DateTime('first day of this month');                                                                        
                                                                                                                      
// correct output: "2011-04-01 22:41:17"                                                                                                   
var_dump($dt);                                                                                                        
                
// wrong output: "2011-04-01 22:41:17"                                                                                             
var_dump($dt->sub(new DateInterval('P2D')));                                                                          
                                                                                                                      
// correct output: "2011-03-30 20:41:17"                                                                                       
var_dump(date_create('@' . $dt->getTimestamp())->sub(new DateInterval('P2D')));