Overstock identifies where your store has more inventory than is needed to meet demand. Excess stock is a factor of the forecast to cover the lead time and days of stock you've set for each variant. Items forecast to have excess units available after your planning period (the end of the lead time + days of stock) will be identified as overstock.
The date range should be set to show historical metrics like sales and revenue for a certain amount of time. The date range in the Overstock report will not affect the forecasted units that will be on hand after the planning period.
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.