start-date-not-exists
$additional_meta_query = array( 'relation' => 'AND', array( 'relation' => 'AND', array( 'key' => '__start_date', 'compare' => 'NOT EXISTS', ), ), array( 'key' => '_end_date', 'compare' => 'NOT EXISTS', ), );
$additional_meta_query = array( 'relation' => 'AND', array( 'relation' => 'AND', array( 'key' => '__start_date', 'compare' => 'NOT EXISTS', ), ), array( 'key' => '_end_date', 'compare' => 'NOT EXISTS', ), );