Supply Plan is generated based on the Demand Plan's Receipt Date. Order Date which is the basis for creating Purchase Orders (or Work Order if you are using Assembly Items) is calculated based on the Demand Plan's Receipt Date and the Item's Lead Time. Therefore, if you intend to make sure that the Supply Plan will not have a backordered date, then Lead Time for the Item should be set to 0. To illustrate:
Assume that today is 06/18/13 and you have a demand tomorrow (06/19/13) for 50. On the Item Record, you have set Lead Time = 5 days. Therefore, the Order Date will be 06/13/13 (computed as 06/18/13 less 5 days) which is a date in the past.
Another illustration is if you are using Assembly Items. Note that Work Order Lead Time is per Quantity. On the Assembly Record, you have set Work Order Lead Time = 5 days and assume that today is 06/18/13. Then, you have demand tomorrow (06/19/13) for 50. Therefore, the Order Date will be 10/11/12 which is 250 days earlier than today. This is because it will take you 5 days to build one Assembly and you require a quantity of 50. Therefore, 5 days * 50 quantity = 250 build days.