Custom query for custom post_type

change your query a bit:

$todays_date = time();

$args = array ('post_type'      => 'tasks',
               'meta_key'       => ''end_date,
               'meta_compare'   => '<',
               'meta_value'     => $todays_date,
               'orderby'        => 'meta_value'
              );

$tasks = get_posts($args);

and then for each one of this posts get the start_date meta and compare with $today something like:

$today = strtotime($today);
foreach ($tasks as $task_post){
  $start = strtotime(get_post_meta($task_post->ID, start_date, true));
  if ($today > $start){
    //to task stuff here
  }
}

Hope this helps.