les dates objets

bonjour,
je ne comprend pas cette exemple
Code:

1 2 3 4 5 6 $d = new DateTime(); print_r($d); echo "<br/>"; $d->setDate($d->format('Y'),10,$d->format('d')); print_r($d);
resultat

DateTime Object ( [date] => 2013-03-14 16:05:48 [timezone_type] => 3 [timezone] => Europe/Paris )
DateTime Object ( [date] => 2013-10-14 16:05:48 [timezone_type] => 3 [timezone] => Europe/Paris )

le 0 à la place de 10

DateTime Object ( [date] => 2013-03-14 16:06:40 [timezone_type] => 3 [timezone] => Europe/Paris )
DateTime Object ( [date] => 2012-12-14 16:06:40 [timezone_type] => 3 [timezone] => Europe/Paris )

le 0 à la place de 24
DateTime Object ( [date] => 2013-03-14 16:07:11 [timezone_type] => 3 [timezone] => Europe/Paris )
DateTime Object ( [date] => 2014-12-14 16:07:11 [timezone_type] => 3 [timezone] => Europe/Paris )

ou bien 1

DateTime Object ( [date] => 2013-03-14 16:07:57 [timezone_type] => 3 [timezone] => Europe/Paris )
DateTime Object ( [date] => 2013-01-14 16:07:57 [timezone_type] => 3 [timezone] => Europe/Paris )

merci de m' éclairer :)