[jdh]

Unless I'm missing something here, why would you want a date relative to completion? If it's to repeat the task, then that's already there in the repeat settings in the inspector (start again, due again and repeat every).

That said, the ability to specify a relative date for start and due dates would definitely be a good idea provided the relative date is stored as a formula/calculation and not just converted to a static date. While entering "S+3d" might be a handy shortcut, I can generally do that in my head so there's no that much point. On the other hand having a scenario where adjusting the start date could also adjust the due date would be a useful feature.