PHP DateTime :: format（'U'）从现在开始返回时间戳，而不是从指定日期返回？

I have a DateTime object, initialized with a specific date:

$inputString = '06102013';
$myDate = DateTime::createFromFormat('mdY', $inputString);

Now I want to give it out in different Formats, using

$myFormat = 'd.m.Y';
$myDate->format($myFormat);

This works perfectly fine for all formats up until the point where I say:

$myFormat = 'U';

Then $myDate->format('U'); contains just the current timestamp (the actual time when the function is called). But how do I get the timestamp from the original input date, $inputString in my case?

If this is important: I'm using PHP 5.3, but plan on upgrading soon.

Edit: Forgot to mention that

$myDate->getTimestamp();

also returns the time of calling, not the original time that the DateTime object was created with!