The advanced due date options can be set whenever you are creating or editing a task that has a due date. Here’s what the options do:
Due BY the Due Date: This is the default. It simply means that the task must be completed on or before the due date.
Due EXACTLY ON the Due Date: This means the task can only be done on the due date, and not before. A good example would be a doctor or dentist appointment. By default, tasks with this option will not appear in your hotlist unless the task falls on the current day (or has a high enough priority).
OPTIONALLY Due on the Due Date: For these, if the date passes and you have not completed the task, the task will be automatically removed, or moved forward if it is a repeating task. By default, tasks with this option will not appear in your hotlist unless the task falls on the current day (or has a high enough priority). For a repeating task, you can combine this option with a task reminder to be reminded about a task at regular intervals, even if you don’t mark the task as complete.
Note that Toodledo also has a “due after” option to specify that a task an only be completed after the due date. Ultimate To-Do List does not implement this, since the start date field can be used instead.