The answer lies in the app's philosophy - they advise "Decide when to get started" on their guide to productivity and then to stick with it. It ties into the larger GTD (Getting Things Done) approach to productivity. To put it very simply, when you specify the start date of a task, you tell Things you don't want to consider working on that task before that start date. After the start date arrives, the task will become visible to you and you will decide in your daily review whether to begin working on that task or reschedule. This does not mean you cannot create tasks that you don't know when you will start - you can create them under Anytime or Someday lists.