The double slider at the bottom of the screen controls the time range of the context widget over which time navigation and search operations will be performed.
The full span of the double slider covers the period between 2010 and the present day. The selection defaults to the last 6 months of available data. The double slider can be adjusted on both sides to select any time period within the full available history of the active measurements.
For a more precise range selection, you can also click the double slider to open a date picker popup.