Description:
------------
A format string containing a Unix timestamp at any point after a microsecond value 
leaves the microseconds unset in the resulting DateTime object.

Test script:
---------------
<?php

date_default_timezone_set('UTC');

var_dump(DateTime::createFromFormat('u U', '123456 123456790')->format('U u'));

?>

Expected result:
----------------
string(16) "123456790 123456"


Actual result:
--------------
string(16) "123456790 000000"