Example: Let's say you want to change your sheets ideally every 7 days but no more than every 12 days. How do you set this up in Marvin?

Go to the "Recurring items" overlay in the top right menu of your day view.

Then add a recurring task with the leftmost button at the top.

Depending on the repeat pattern you need for your task you select a different repeat option:
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If the time window for a task is specific to a day in a month or week use the "every month" or "every week" pattern.

If the time window repeats in a regular interval regardless of when you actually completed the last task, use the "every X days" pattern.

If the time window is specific to when you last checked off the task use the "X days after completion" pattern.

To give yourself a specific window of time where you can do the task you specify the first day you can do the task as part of your repeat pattern. In our example, this would be "repeat every 7 days after completion". This ensures that the task shows up on the first day you can do the task, which is 7 days after you last changed your sheets.

To set the last possible day you can do the task (the end of your window) you add an automatic due date to the recurring task. You can specify a specific number of days after the task creation (the first day of the window) for a task to become due. 

So if you have a 3 day window to do a task, you would give it an automatic due date 3 days after creation.
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In our "change sheets" example we would specify a 5 day window (12-7 = 5) instead.