How to use negative and positive days in Dynamics AX
When Dynamics AX master planning is rolled out to new users two of the settings that generate much confusion are “negative days” and “positive days”. These two fields are found on the Master plan coverage group form. It is important to understand these fields and tune them to your requirements in order to receive the results from the planning engine that you expect. These fields are quite powerful and useful in that they control when in the future time line the planning engine will recommend purchases.
Let’s talk about Positive days first. This field determines how far into the future the planning engine will consider current inventory to fulfill future demand. For example; if you have insufficient inventory on hand today for a fast moving item that has short lead time (one week long) and you receive a sales order for that item with a delivery date far into the future (six months out), do you want to have the system consider that inventory for the sales order now and recommend a purchase? You may not because any inventory available today will be long gone before it is required for the sales order and there is plenty of time to purchase when the requirement is closer. If your positive day setting is greater than six months the system will not recommend that you purchase, if the positive day setting is less than six months the system will generate a purchase.
This setting will also impact action messages. Taking the same example from above and adding to that example sales order with a requirement one month from today and a matching purchase order with a just in time delivery for that sales order. Do you want the system to generate an action message that recommends that the existing purchase order be increased to cover the sale which is six months away? If the positive day setting is greater than six months the system will recommend that you increase the quantity on the existing purchase order. If the positive day setting is less than six months the system will not generate such a message.
For ease of use I suggest a parameter setting of about 45 days. In our example the system will not generate an action message to increase the quantity of the existing purchase order and will instead generate a future purchase one week before the requirement is due.
Let’s talk about negative days. This setting determines how late we will allow this item to be. Let’s say we have that same item with a sales order requirement due twenty one days from today. We also have a purchase order providing plenty of inventory in the system with the receipt expected twenty four days from today. Do we want the system to tell us to create a new purchase, or to allow the sales order to be late by three days and just use the existing purchase order? If negative days is set to less than three the system will generate a new purchase and an action message suggesting that you expedite the current purchase order, the operator has a choice of doing either. If the negative day parameter is set to greater than three the system will do nothing and assume that you will fill the sales order a few days late. I recommend a setting of about less than three days.
Since these settings are controlled at a coverage group level and one size does not have to fit all, it is important to carefully analyze your items and their lead times. You should categorize your items and group them into coverage groups so that the system will make the recommendations that you expect for each type of item.