D8 - How to modify url params on form submit?
You might have a logic-error in 

$input['end-date'] = $start_date;


should be this:?

$input['end-date'] = $end_date;


How ever you can add a parameter with "query->add":

\Drupal::request()-&gt;query-&gt;add(['start-date' =&gt; $start_date, 'end-date' =&gt; $end_date]);


Note that you "mix" POST and GET - variable. So in your "submitForm" - method you need to process the GET - variables by yourself.