$view           = views_get_view('my_view');
$view->set_display('default');

$date       = $view->get_item('default', 'filter', 'created');
$date['value']['min']   = date('Y-m-d', $date_from);
$date['value']['max']   = date('Y-m-d', $date_to);
$view->set_item('default', 'filter', 'created', $date);

$view->pre_execute();
$view->execute();
$results = $view->result;

$view->destroy();

// do logic here to determine if view needs to be queried a second time

$view           = views_get_view('my_view');
$view->set_display('default');

$date       = $view->get_item('default', 'filter', 'created');
$date['value']['min']   = date('Y-m-d', 0);
$date['value']['max']   = date('Y-m-d', $date_to);
$view->set_item('default', 'filter', 'created', $date);

$view->pre_execute();
$view->execute();
$results2 = $view->result;

$view->destroy();

// $results and $results2 are identical