How to create better WP_Query to look for date time which is anywhere between two meta values?

You might want to do this for querying between two dates:

$query = new WP_Query(array(
                        'post_type' => 'rezervacija',
                        'posts_per_page' => -1,
                        'post_status' => 'publish',
                        'meta_query' => array(
                                'relation' => 'AND',
                                array(
                                    'key'     => 'start_date',
                                    'value'   => current_time('Ymd'), //change this according to your need.
                                    'compare' => '>=',
                                ),
                                array(
                                    'key'     => 'end_date',
                                    'value'   => current_time('Ymd'), //change this according to your need.
                                    'compare' => '<=',
                                ),

                            ),
                    ));

    if ($query->have_posts()) : 
        while($query->have_posts()) : 
            $query->the_post();
            the_permalink(); 
            the_title();
        endwhile;  
    endif;