At the bottom of the page where you "save" or "assign" you should see "Assignment will post immediately with late turn -ins allowed. Edit"
If you click edit, you're presented with the timeline screen
Here you can select "schedule to assign in the future. You are also allowed to select a "close date"
Post Date: When it becomes available.
Due Date: When it's due
Close Date: When it will no longer be available. (If this is the same as due date, then no late turn-ins are allowed. If this is after Due date, then students will be allowed to turn in late, but their submission will be marked "LATE" and you''ll see that they did.