Conversion between fiscal and normal calendar

Good afternoon!

Our fiscal year is not based on calendar months - we are using the 4-4-5 week model. We have recently implemented Opportunities in SAP ByDesign and this being a non-transactional activity, only the normal calendar is available for reporting. Our Sales team however needs to be able to report opportunities per fiscal period. I cannot find any data sources that would allow me to convert the calendar date to the fiscal period and add this to the Opportunity Pipeline report. Would anyone how could I create this conversion?