Labs for class

Hello, this is my first time using Power BI. I need to find the sales variance percentage using this formula Sales Var % = DIVIDE([Sales Var],[LY Sales]). Also, I used this formula LY Sales = CALCULATE([Total Sales],SAMEPERIODLASTYEAR(‘Date’[Date])) I keep getting zero because the last year ends at 2002. What I am trying to find is the sales var and sales var % with my data. I have last years sales, but the sales var keeps giving me zero. Any suggestion on what to do? How do I specify in dates? Thanks in advance!