About Recurring Tasks
Since GoodTask uses and shares data from default Reminders app, recurring tasks are managed by iOS and macOS. GoodTask sets up recurring rules just like Reminders and connects it on the task.
How Recurring Tasks work on default Reminders App
You basically set up a task with due date and select recurrence rule for the task. (i.e Every day, Every month, Every last day of the month, etc)
When you complete that task, next task will be made based on that rule. So even if you missed the task for some days, next one will be made on the future according to that rule.
Advance features in GoodTask
Since default Reminders only take care of alert time (reminder), it doesn't care about start date and due date that you can use separately on GoodTask. This makes the recurring task behave not as expected. GoodTask runs it's own engine before and after next task is made to make it match the differences..
You can have check 'Repeat on Completion' option which is only available on GoodTask. If you set 'Repeat every 2 weeks' for example and you complete that task, normal behavior is next task will be made based on due date that was set. This means if you made a task due tuesday and get it done on thursday, next one will be made on tuesday 2 weeks later.
If you have 'Repeat on Completion' option turned on, next one will be made on thursday 2 weeks later. This will only work when you complete the task on GoodTask.