searching between dates

I am having some trouble with one of my advanced custom fields. I have a custom post type that I have added some ACFs to. The post item is a news item with various custom fields (one of which is called news_date) – the date is stored as yyyy-mm-dd and perhaps this is where I am going wrong. I am having trouble finding an example to work from but my code is as follows and it should theoretically pick up the 4 news items that i have in there.

$start = '2010-01-01';
$end = '2013-12-31';
$args = array(
    'post_type' => 'news',
    'posts_per_page' => -1,
    'orderby' => 'meta_value',
    'order' => 'ASC',
    'meta_key' => 'news_date',
    'meta_query' => array(
        array(
            'key' => 'news_date',
            'value' => array($start, $end),
            'compare' => 'BETWEEN',
            'type' => 'DATE'
        )
    )
);

$news_query = new WP_query();
$news_query->query($args);

This is giving me the following SQL:

SELECT wp_posts.* FROM wp_posts INNER JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id)
INNER JOIN wp_postmeta AS mt1 ON (wp_posts.ID = mt1.post_id) WHERE 1=1 AND wp_posts.post_type = ‘news’ AND (wp_posts.post_status = ‘publish’ OR wp_posts.post_status = ‘private’) AND (wp_postmeta.meta_key = ‘news_date’
AND (mt1.meta_key = ‘news_date’ AND CAST(mt1.meta_value AS DATE) BETWEEN ‘2011-11-31’ AND ‘2011-10-01’) ) GROUP BY wp_posts.ID ORDER BY wp_postmeta.meta_value ASC

Any advice on what I may be doing wrong would be much appreciated.

Thanks.

http://wordpress.org/plugins/advanced-custom-fields/