Description:
------------
Using the current datetime, adding and subtracting one month does not result in an actual one month addition and subtraction

Test script:
---------------
        <td valign="top">
	    <?php

	    $previousMonth = new DateTime ('2013-03-31');
	    $previousMonth->sub (new DateInterval ('P1M'));
	    echo $previousMonth->format ('Y-m-d');

	    ?>
        </td>

        <td valign="top">
	    <?php

	    $nextMonth = new DateTime ('2013-03-31');
	    $nextMonth->add (new DateInterval ('P1M'));
	    echo $nextMonth->format ('Y-m-d');

	    ?>
        </td>


Expected result:
----------------
previous month to show 2013-02-*, next month to show 2013-04-*

Actual result:
--------------
<td>2013-03-03</td><td>2013-05-01</td>