Opening Stock

Hi, I created a Stock report based on date. I want to have opening balance based on either year/month selected. Logic would be, if user has selected the Jan 2015, then opening balance should be sum of all transaction till DEC 2014 . I am not sure how to model in HANA. However it doesnt work as expected. Appreciate if anyone had handled this earlier, would provide insight. Note: We are not BW customers Regards Ben