You can select Drive audit – Apply custom filter – Updated before or equal to July 1, 2015
This will display only the files across your domain that last been updated before July 2015
To do this you would need to use GAT Unlock
Once You or the new owner becomes the owner you can delete the files.
The files will be accessible on your Google drive.
Here is a link you can follow for GAT Unlock feature if you need.