Show post when between two dates

Hi everyone!

I’ve made (with Advanced Custom Fields) a date picker where you can choose from which to which date you want to show a post on the site. For example I want a post to show from 01-01-2014 to 01-01-2015 and after the last date the post won’t be shown anymore.

I’ve found a hack with the where filter, but I can’t seem to figure out how to use that function with variables.

<?php
// WP_Query arguments
$args = array (
	'post_type'              => 'vacature',
	'pagination'             => false,
	'posts_per_page'         => '3',
);

$start = get_field('online_vanaf');
$end = get_field('online_tot');

function filter_where( $where = '' ) {

global $start;
global $end;

$where .= " AND post_date >= '$start' AND post_date < '$end'";
return $where;
}

add_filter( 'posts_where', 'filter_where' );

$vacatures = new WP_Query( $args );

// The Loop
if ( $vacatures->have_posts() ) {
	while ( $vacatures->have_posts() ) {
		$vacatures->the_post();

?>

You can see my code above. I don’t get an error, WordPress just won’t show the posts anymore..
Any help is welcome!

Thanks in advance.