For reference, the generated SQL is:
string(640) "SELECT SQL_CALC_FOUND_ROWS wp_posts.ID 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) INNER JOIN wp_postmeta AS mt2 ON (wp_posts.ID = mt2.post_id) WHERE 1=1 AND wp_posts.post_type = 'event' AND (wp_posts.post_status = 'publish') AND (wp_postmeta.meta_key = 'x_start_date' OR (mt1.meta_key = 'x_start_date' AND CAST(mt1.meta_value AS DATE) >= '2014-02-24') OR (mt2.meta_key = 'x_end_date' AND CAST(mt2.meta_value AS DATE) >= '2014-02-24') ) GROUP BY wp_posts.ID ORDER BY wp_postmeta.meta_value ASC LIMIT 0, 5"
