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
?>