To modify milliseconds and microseconds use next formats:
<?php
$a = new \DateTimeImmutable('2018-01-01 00:00:00.000');
$b = $a->modify('9 msec');
var_dump($b->format('Y-m-d H:i:s.u'));
?>
Returns string(26) "2018-01-01 00:00:00.009000"
<?php
$a = new \DateTimeImmutable('2018-01-01 00:00:00.000');
$b = $a->modify('9 usec');
var_dump($b->format('Y-m-d H:i:s.u'));
?>
Returns "2018-01-01 00:00:00.000009"