ok, i was using the wrong class.
i did:
$date = new Date();
$date->setValue($this->Created);
return $date->Ago();
But the method setValue() of the Date-Class is creating a Date in format "Y-m-d".
So hour, minute and second of $this->Created are considered as 00:00:00.
Instead I had to use the class SS_Datetime
$date = new SS_Datetime();
$date->setValue($this->Created);
return $date->Ago();
maybe this is useful for anyone.
One more notice: In german it should be formulated "vor 9 Tagen".
Mayby this is diffucult to do with translation, because "Tage" is the correct plural for "Tag".
But in this case it has to be "vor 9 Tagen".