How is the delivery date calulated in case of Stock transffer Order

Dear Gurus

I would like to know how the delivery date calulated in case of inter company stock transfer order.

Normally it should check the Planned delivery time from the inforecord/PDT from material master .So Delivery date = Po creation date +PDT.

But this rule in not consistent if I change no of days, its calulating in diffrent way.So what are the other factors which will be considered here.

My factory calendor-All are working days including sat/sun, Gr proceesing time is 0 and Purchasing time is also zero days set.

Thanks for feedback

Regards

Amar