I'm sorry that you encounter this issue. There are two ways to do this filtering. Unfortunately neither of them could do exactly what you want.
One way to filter the data is by the "Last Modified" field and status Completed. However this does not ensure you that this item is completed within a seven days because last modified change could be other then the status.
The other way is the one that you said. You need to create a custom field and set it to done date, when you move the item to status "Completed".
I hope that the information helps you. Feel free to write us back if you need further assistance.