How Do IModify Report Criteria?
You may want to first check out the Advanced Fundraising Reports extension before you go too far down that road - I think it has the option you want.

Second - I've been working on user-configurable date filters - I'm hoping to get them into the next version of Civi, so you don't have to modify code to get a new filter.

Third - since you need this now and not later - you'll want to override one or both of these core files: 
CRM/Core/Form/Date.php 
CRM/Utils/Date.php (function relativeToAbsolute).