A Power BI Date slicer is a graphical user interface (GUI) element in the Power BI dashboard that allows the user to filter data by date ranges. It is a visual filter that can be used to select a specific date or a range of dates to display data for. The slicer can be used to filter data in a table, chart, or other visualization in the dashboard. The user can select dates using a calendar view or by using predefined date ranges such as "Last 7 Days" or "This Month." The slicer can be customized to show dates from a specific data field or to display a custom list of dates. It is a useful tool for analyzing and visualizing data over time.