Users can set a due date when creating a new task or by modifying exsiting tasks.

From My Day:

      1) Right click and select Edit Task. 
      2) Change the Due Date and click SAVE.

 From Tasks:

     1) Double click on the task to pull up the edit menu. 
     2) Modify the Due Date and click SAVE.

 From Calendar:

    1) Right click on the task and select Edit Task.
    2) Select the Due Date and click SAVE.