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;