At the moment, if you want to have a sale, you have to email a code to your customers, or include it in an article on your site.
I would suggest that a global modifier (or set of Global Modifiers) would be a nice option.
Instead of creating a promo code to be entered by customers, have a shop promotion that is automatically applied to all or selected goods (as per my previous post on improved Promo Codes:
http://www.ijoomla.com/option,com_smf/Itemid,6000/topic,1602.0/), and that will expire at the exact date and time specified. Additionally, the reduction would be shown, i.e. "Original Price:
$80 Now Only $60!
So, for example, you have a Shoe Store.
You could have a special 30% reduction on all Ladies Boots that lasted a week.
Or, better yet, you could create a "Reduced Category" where the reductions are applied by that category - or even different reductions by Sub Category:
Top Level Category: Reduced
Subcategory Level 1: Reduced by 10%
Subcategory Level 2: Ladies Shoes
The 10% reduction would cascade to each Subcategory Level 2.
Then you could also have Subcategories Level 1 like "Reduced by 15%" or "Reduced by 50%" or whatever you like.