Comment 39 for bug 307680

(In reply to comment #20)
> I propose the following workaround. A recurring task should be treated as a
> standard one except that when it is completed, its recurrency is examinated: if
> more instances have to take place, a new (recurring) task is automatically
> created. This means, more or less, that a recurring task will have a "clone" for
> each recurrence.

This is basically the way that Outlook behaves. When you create a repeating task only one task is created. You can choose to repeat in the normal manner (pay rent on the first of every month) or based on the time of completion (water the plants 4 days after you last watered them). In either case, when you check the box to mark the task as completed, a new task is created with the same recurrence as the original task. The recurrence is removed from the original task, and the original task remains as a standard, non-repeating, but completed task. This allows you to keep track of, for example, when you last watered the plants, or that you did indeed pay rent last month.

Although this particular interpretation may not be specified in the RFC for ICS, it seems to me like this interpretation would work with any other program that reads ICS. If you export this as an ICS file, then the completed tasks show up as separate tasks and the remaining tasks are listed as a single recurring task to be interpreted however the new program wants.

In any case, this seems (to me at least) to be an important feature lacking from Calendar. I just attempted to migrate my Outlook task list to Sunbird, and found that the majority of my tasks are repeating tasks and couldn't be properly entered into Sunbird.