Issues with statistical modeling in calendar months vs 13 fiscal periods

In our demand planning, we plan at the fiscal period level (13 fiscal periods across a year - not matching with calendar months or year). We are having issues (strange results) trying to run the statistical modeling at the fiscal period level versus the calendar month level. Is there a way to run the stat modeling over fiscal periods rather than calendar months? Can we run stat modeling over a specific date range rathn than including the current month?