You can edit the expiration date after you added a product to your stock.
From Inventory -> Documents -> Supplier Invoices you can edit the reception note and access the specific product.
Don't forget to save the product changes and then to SAVE the reception note!
Q: How can I find out the reception number easily?
A: Access your stock page and see the details by clicking on the purple arrowhead. Reception note number is displayed in the first column.