The quickest way to change the dates in the grid is to select from a list of already defined periods. Defined periods contain a current period and previous period and are usually dynamic, meaning they adjust according to today's date.
In Summary view or when you have selected a dimension (as applicable), click the Period menu, then select a defined period from the list.
When the grid updates with the new defined period, you can check what dates that period refers to: click Period > Custom. The Custom Period window displays the date range.