If I don’t finish my inventory on the same day I start, when I go back to complete it, do I need to alter the Start Date for the session I created?

No, you do not, and most likely wouldn’t want to. When you scan an item to be inventoried, an Inventory Date is attached to that item. The date attached to those items is the actual day you scanned them. If you alter the Start Date for your inventory session, than your inventory session would not include the items you had scanned on previous days. Your inventory sessions can last as long as you need them to. Don’t worry if it goes past a day, a week, or even a month.