DateTime constructor in php

DateTime constructor in php

I have done a little experiment on php's DateTime class. In the
documentation they have suggested the syntax for creating the object of
DateTime class as given.
Object oriented style
public DateTime::__construct() ([ string $time = "now" [, DateTimeZone
$timezone = NULL ]] )
Procedural style
DateTime date_create ([ string $time = "now" [, DateTimeZone $timezone =
NULL ]] )
Returns new DateTime object.
Here the first argument they have specified as mandatory, as a date/time
string as specified in Date and Time Formats section. Or we have to pass
NULL here to obtain the current time when using the $timezone parameter.
But my problem is when I am giving following code -
date_default_timezone_set('America/Los_Angeles');
$d_ob = new DateTime('x');
echo $d_ob->format('Y-m-d');
It's supposed to generate exception, but it's generating the echoing the
current date time like -
2013-09-29
I am not getting the point, how it's working?