With Published / Unpublished dates you can set products which will be published between those two dates ( like promo products ).
To activate this future have to add two shop custom fields:
1. published_date (field type: datetime )
2. unpublished_date (field type: datetime )
After Upload your project changes and Click on "Update database" button in shop Online Administration "settings" section you will see
PUBLISHED_DATE / UNPUBLISHED_DATE row when edit or create new product. (In order to work, "PUBLISHED" checkbox of your product must be enabled).
First date is published date and by default it gets value of the created or updated date of the product.
By default unpublished date is not set so product will not have expired date. To activate unpublished date click on checkbox "unpublish":
Now all you have to do is set proper dates and this product will be published only in this period PUBLISHED_DATE - UNPUBLISHED_DATE.