克隆的DateTime更改值

Question

I have a value, a DateTime that I clone and without touching it in a foreach I display it again. But without any clue the value has changed. I know it's from the:

if ($rtt->getPrttDateDebutPose()<$conge->getCngDateDebut() || $poseRTTService->getDateLastDay($rtt)>$conge->getCngDateFin()) {

Because I tested it, it's what can possibly change the value (even though I can't figure why either). But a clone ? Why would it change I don't do anything with it ?

Here's the complete code :

$end_rtt = clone end($RTT_pris);
var_dump($end_rtt); // -------------------------------------------------------------------------------------
// Algo that allows to separate in two arrays the conges w/ rtt and those w/o
foreach ($conges as $conge) {
    $count = 0;
    foreach ($RTT_pris as $rtt) {
        if ($rtt->getPrttDateDebutPose()<$conge->getCngDateDebut() || $poseRTTService->getDateLastDay($rtt)>$conge->getCngDateFin()) {
            $count++;
        }
    }
    if ($count == count($RTT_pris) && !in_array($conge, $conges_final)) {
       array_push($conges_final, $conge);
    } else {
        array_push($conges_w_rtt, $conge);
    }
}
var_dump($end_rtt); // ---------------------------------------------------------------------------------------

And here's the output of the two var_dump($end_rtt) :

See how the date pass from 11 to 20.

If anyone understands that, do please explain.