Description:
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The DateTime class does not seem to update the object properly, specifically when 
the 'time' specified in the constructor of the object is relative time format 
(such as `first day of this month`). The test case is reproducible on PHP 5.3.6 
as well as PHP 5.3.2 using the test script I've included below. A DateTime::diff 
after using a DateTime::add method on the object results in 0 for all periods 
(when the period specified with DateInterval is in Days). This clearly is not the 
expected behavior as using PT24H will increase the object date by one day, but 
not result in objects date being updated, while P1D (or P1W - which translates to 
days) will have no effect at all.

Test script:
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	$timezone = new DateTimeZone(date_default_timezone_get());
	$times['relative1'] = new DateTime('first day of this month', $timezone);
	$times['relative2'] = new DateTime('-1 week', $timezone);
	$times['fixed'] = new DateTime('4/1/2011 12:00:00 AM', $timezone);
	$intervals['PT24H'] = new DateInterval('PT24H');
	$intervals['P1D'] = new DateInterval('P1D');
	$intervals['P1W'] = new DateInterval('P1W');
	$intervals['P1M'] = new DateInterval('P1M');
	$intervals['P1Y'] = new DateInterval('P1Y');
	echo "<pre>";
	foreach ($times as $type => $time) {
		foreach ($intervals as $period => $interval) {
			$str = "From " . $time->format('m/d/Y H:i:s') . " To ";
			$last_time = new DateTime($time->format('m/d/Y H:i:s')); $time->add($interval);
			$str .= $time->format('m/d/Y h:i:s') . " - PERIOD: $period";
			echo "<b>Using <i>$type</i> $str</b>\r\n";
			echo $time->diff($last_time)->format('%y years, %m months, %d days, %h hours, %i minutes, %s seconds.') . '<br />';
		}
	}
	echo "</pre>";

Expected result:
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Specify a DateTime object using relative time format of 'first day of this month'

DateTime object updates by 1 day when using DateTime::add() with a 
DateInterval('P1D') on that object.

Actual result:
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DateTime object does not update even though the DateInterval is valid and works 
on other DateTime objects of different, or non-relative, time.