FOB Foreign Port - How to account for intransit time

We order parts from a vendor in China.  The total leadtime is 12 weeks.  The FOB terms are Foreign Port and the vendor is only responsible to ship the parts to the port within 6 weeks of receiving our Purchase Order.  We need to have the PO reflect the FOB date because that is the legally binding date and vendor performance is based on that date.  But we also need the PO to reflect the 12 week date so planning does not ‘under plan’ (because, with the FOB date as the PO date, planning will think the parts are coming