I guess the start date should indicate the date when you plan to manage the task, the lenght should indicate the time you will take to complete the task. Both start date and lenght should synchronize with the calendar, to let you visualize for each day the tasks you have planned and how long will they last. In case You are not able to plan a date to complete the task, you will simply indicate the due date, to remind you that the task has to be managed.
I consider the due date as the deadline of the task, in this way the allarm is the tool to remind you to manage the task in case you haven't been able to in the start date you planned.
If due date represents only the end of task and synchronizes with the length, it would be a double field for the same feature, and would be less useful. In my opinion a tool to plan daily activity would be much more interesting.