The schedule date on the outbound delivery is the requirement date that is the basic for the other dates in the supply chain.  

Before you confirm the sales order you can manually set a date as delivery date in the sales order. This will override the Customer lead time you have set in the product, and the schedule date in the outbound delivery will be accordingly. (+ the security lead time you have find in Inventory->settings.)

After sales order confirmation, you can manually modify the date in the outbound delivery.  If a manufacturing order, picking and purchase orders already have been created, you must manually maintain the dates there accordingly.