Sales Forecast includes Unbilled Orders based on incorrect date

I've looked everywhere I know to look and can't seem to find an answer to this.

We take a large number of orders with forward ship dates.  In any given month we take orders that ladder out ship dates as far as 10 months forward.  These orders are live and binding, as we manufacture to order.

The Unbilled Orders section of the Sales Forecast only seems to includes orders by the Sales Order Date (creation date), when those orders won't actually be billed until fulfillment, which is on or after the Expected Ship Date.  The resulting Monthly Sales Forecast is understated by the amount of any orders shipping this month but written/created in previous months and overstated by the amount of any orders written in the current month but not scheduled to ship until future months.