If, like me, you don't want that information into a table but want to display it in some HTML or in the title of panels, edit the xml and locate the ... block. Witin it add a "done" clause like in this example:
<p>Showing data from $earliest_token$ to $latest_token$.</p>
There is many ways this can be tweaked to your preference, starting with the format you give to strftime. This example assumes that the earliest/latest are relative times such as "-7d@d" etc. You'll need to update the eval if they are epoch timestamps for instance.
Also, I'm using this with a base search at the top of the dashboard (outside any panel). If you're trying this with a search within a panel, I'm not a 100% sure the tokens will be available everywhere in the dashboard.
One slight annoyance with this solution is that the earliest/latest tokens are not populated until the search is done, but I haven't found how to avoid that. Works for me anyway.