O_CUSTOM_PRICE_PUB. GET_CUSTOM_PRICE_DATE

I would like to use the API PO_CUSTOM_PRICE_PUB. GET_CUSTOM_PRICE_DATE to change which date determines our Price Breaks.  In the Purchasing API documentation, it states that this API can be used to determine the pricing date, which can be "the date that the order is placed, the date that the order is shipped, or the date that the material is expected to arrive"(the last one being the one we want to use).  By default it uses the Need-By-Date.  We want it to be based on the Promise Date, but I'm not seeing any obvious way to do this, so I wondered if anyone out there had any suggestions?