In the enterprise resource planning literature, time fences have been suggested as a method to control forecast effects on resource decisions. Time fences act as a trigger to ``firm up'' the resource plan against the forecast and serve as a mechanism to freeze the schedule to ensure no changes are made that will impact specific resources. The immediate time period fence (product time fence) specifies the time when material may be on order and schedule changes should not occur. Schedule changes that occur inside the product time fence are likely to cause material shortages or excess inventory. The short time period fence (demand time fence) specifies the time where demand drives the capacity requirements