PHP DateInterval Problem

I'm trying to get estimated delivery dates for various products but rather than working it out for each product it adds the previous delivery time onto the next product before working it out.

This is what I've got so far

[code]

public function getDeliveryDate() {

       $ndd = $this->getPhysicalOrderProducts();

        $arrive = $this->getPostDate();

        $arrive->add(new DateInterval('P'.$ndd[0]->getDeliveryTime().'D'));

        return $arrive;

      }[/code]

I want it get a product, add the DeliveryTime to PostDate then return that as $date I then want it to go on to the next product and do the same thing. At the moment what's happening is it's getting the date, adding the delivery time. Then with the next date it's adding the previous delivery time to the result of $date from the previous product.

Is there anyway to get it to recalculate it from fresh for every product?