Yeah, it´s not ritght : )
This expression offset is set in years, so its returning the ytd sales for the last year!
You can make calculations with dates like integers:
sum(if(inYearToDate(Sales_Date, (today()-1) ,0), Value)
For expressions that needs an offset of weeks, months there is a group of functions (addmonth(), addyear(),etc) that does the job!
Hope it helps!