First of all, for general understanding.....if you hover over a bullet it will show you "last changed", which also is applicable to items that are created, modified, or completed. Therefore, by using special search operators, you can view items that were completed within a certain number of days.
First, you must ensure that your setting to hide completed items is set to visible.
You can then use any of the following searches.....
This will show you all completed items
This will show you all completed items within a given time frame (i.e. 7 days).
This will show you items that were "changed" in the last 7 days (will include create, modified, or completed)
Here is a blog article on our hidden search operators.....
Using these searches in tandem with the "last changed" date stamp that shows when you hover over an item is very powerful.