PowerPivot is a superb addition to Excel: it allows you to create pivot tables based on multiple tables taken from a variety of data sources, and its DAX language lets you report any statistic you want. This overview summarises how to get started with PowerPivot, and what it does.
This blog shows you how to create a calendar of dates for use in PowerPivot. Instructions are included for Excel and Access.
This blog includes the SQL script to generate a table of calendar dates for linking to from within PowerPivot.
PowerPivot for Excel allows you to crunch very large amounts of data quickly, using pivot tables and slicers and other familiar Excel concepts. It also includes the DAX language, allowing you to create complex calculations.