Even if you played around with SharePoint calculated fields before I bet you didn`t know this – you can extract date parts from the date field.
More about this and other great examples that will save your day are here:
http://intellects.in/2009/03/04/sharepoint-calculated-columns-formulas/
http://www.experts-exchange.com/OS/Microsoft_Operating_Systems/Server/MS-SharePoint/Q_24229126.html
