Extending The JavaScript Date Object with User Defined Methods
http://www.digital-web.com/articles/javascript_date_object_with_user_methods/

O'Sullivan, Lawrence
Digital Web Magazine
2008

Abstract:

Applications, particularly business applications, can often require a lot of date manipulation, but that code could be simplified if JavaScript’s core Date object had some additional methods. In this article, I will show you how to add custom methods to the Date object that are inherited by each date instance.