This function will not change the value of the DateTimeImmutable object as the method name might suggest. The object, after all, immutable.
<?php
$dti = new DateTimeImmutable();
echo $dti->getTimestamp(); // e.g. 123456789
$dti->setTimestamp(987654321);
echo $dti->getTimestamp(); // 123456789
$x = $dti->setTimestamp (987654321);
echo $x->getTimestamp(); // 987654321
?>