Previous Years vs Current Year

Hi,

I want to compare current year's sales vs last year's sales. I am not 100% clear on how time intelligence features work.

I have a "Years" that has the year, year beginning date & year end date.

   PreviousYearSales = CALCULATE(sum(InvoiceDetail[TotalSales$]), PREVIOUSYEAR(Years[EndofYear]))

I am filter sales out for 2015 and the "PreviousYearSales" come out blank?!

Can some one sum up the logic on how this works?

Thanks,

 Steven