The date you specify for an item in a stock list indicates when the item was added to the stock. By default, Grosh will assign the current date to the field when adding new items. However, you can modify the date using the date picker in the item details screen. This could be done to set a date in the past when the item was actually put in stock.
In future versions, we'll add further functions to the stock date, but for now the app highlights items in red on the suggestion screen with a date that is 6 months or older.