As the type of the first parameter is string, we can assign a relative date for it.
Example:
<?php
$immutable = new DateTimeImmutable('-1 year', new DateTimeZone('Asia/ShangHai'));
echo $immutable->format('Y-m-d H:i:s');
?>
It looks like the parameter of function strtotime and is very helpful for us to get an immutable relative DateTime object.