A few options here, easiest is to just put the sales_date on the column (or row) shelf. Filter the months you want to show and put SUM([number_item_sold]) on your text shelf (or where you want it), then turn on row totals from the Analysis menu -> Totals -> Show Row Grand totals.
Does that help?
So the Grand total would be a sum of the monthly sub-totals displayed in the view (true?), which is determined by the sales_date filter. Let's say I have a three month sales data set. Depending on the sales_date filter (a date range slider), the view would show one, two, or three monthly sub-totals; consequently the Grand total would be a sum of one, two, or three monthly sub-totals. However, I'd need the grand total to always be a sum of the entire three monthly sub-totals, no matter what the sales_date slider sets.