The date range should be set to consider sales history. Sales history during that time will be used to calculate forecast and sales velocity.
Days of stock is the amount of time you want your stock to last. So if you'd like your stock to turnover every 30 days, the overstock will report will use the forecast to show you that after your 30 days of stock, here's how much stock you have on hand.
Read our article about how to update your days of stock.
Note that there are some default filters on that report, so not all variants will show when you first click on the Overstock report. You can remove the filters to see everything.