Setting a dashboard date range means that a specific time frame will be active whenever you or your client open or reload the dashboard. After the dashboard is opened, your client will be able to choose any other time frame for which they want to see the report in Reporting mode.
To set a specific date for your dashboard, you need to be in Edit mode.
Open the General & Style category located in the right sidebar. Click on Dashboard date range to set your desired time frame. Make sure to hit the “Apply” button after you've selected your preferred time frame.