What you can do is to create a date parameter like this. This will be the day the user choose.
Now, If you want to see the sales for 42 days before the date N parameter, you just create another calculated field like this to get the sales between the both date.
IF DateOfTheSales>DATEADD('day',-42,[Date Now] AND DateOfTheSales<[Date Now]
Is it what you're looking for ?