Well, there is now an automatic way to know which was the stock one month ago, but I will explain to you the option you have with ATUM.
- ATUM Action logs
With Action Logs you can track any change in your e-commerce. So, if you search a specific product, you will be able to see its stock changes along the time:
I leave you here the documentation of the add-on where you will find useful info about how it works: https://forum.stockmanagementlabs.com/t/action-logs-documentation
And the link to the demo site in case you want to try the free trial: https://stockmanagementlabs.com/addons/atum-action-logs/
- ATUM Export Pro
With ATUM Export Pro you can export almost any kind of data in your site, and you can programme to make an export daily, weekly, monthly... So you will be able to compare between the current export and the last month export to see the stock changes.
I leave you here the link to the documentation: https://forum.stockmanagementlabs.com/t/atum-export-pro-documentation
And the link to the demo site: https://stockmanagementlabs.com/addons/atum-export-pro/