Discounts in Commerce have generally required configuration using rules, and have often required hand-applied configuration, which is probably out of reach for many site managers. In the Commerce Discount Campaign Feature (drupal.org/sandbox/rfay/1357008) I've put together a few rules with a content type so that discount amounts and dates can be done just by editing a node. This screencast tells how.

This project provides an easy way to do time-limited discounts in Drupal Commerce. A campaign is a time-limited discount that can be applied to any product.

Watch the screencast demonstrating this feature module.

Enable this feature, then

You'll need References, Date, Features and Strongarm
Enable this module.
Add the field "field_campaign" provided in the feature to your product type
Create a node of type "Campaign" with a start date and (optional) end date and a discount (like .1 for a 10% discount)
Enable the campaign on one or more products.
The feature provides

A content type for campaigns, with date and discount fields
A nodereference field to point to campaigns (that goes on the product)
A few basic rules that apply the discount percentage provided in the campaign node when the dates in the campaign node are active.
This is really just a demonstration. You can expand this to offer discounts to roles or anything else you want.

If you don't use git and just want to download it you can grab the tarball. (Oops. The repo viewer's tarball generation seems broken right now. You'll have to get it with git - use the 7.x-1.x branch.)