http://www.infopathdev.com/forums/p/25144/87127.aspx
This is something I've done quite a bit. I end up using SP 2010 calculated columns. As I have to do a fair amount of reporting based on the data entered into a particular form. Here is one way that you could do it - http://tamalijs.wordpress.com/2011/03/10/sharepoint-calculated-date-fields/.
Also - http://bobbyspworld.com/2013/05/31/calculate-fiscal-year-using-workflow/