I’m not a fan of using a 13th period in Dynamics GP. It tends complicate some things unnecessarily. Most folks want a 13th period to be able to isolate and identify audit or post closing adjustments. A 13th period is a way to handle that but there’s a better way.
GP allows us to select different sources for GL transactions. Instead of using the default Source Document of GJ, you can use your own source document with a name like Audit PostClose.
To create your own source, simply pick Administration>Posting>Source Document. Give the source an ID and a description.
When entering an audit adjustment simply change the source document to your new source. This makes it easy to pick out adjustments via SmartLists or other tools.