Echo page title from page with custom meta datetime picker

This should output the title of the page with the most recent timestamp:

$args = array(
    'post_type' => array( 'page' ),
    'order' => 'DESC',
    'orderby' => 'meta_value_num', // Note was `meta_value` before
    'meta_key' => '_cmb_test_datetime_timestamp',
    'numberposts' => 1,
);
$posts = get_posts( $args );
if ( $posts ) echo $posts[0]->post_title;
wp_reset_postdata();

To add conditions, use a meta_query clause, eg

$midnight = strtotime( date( 'Y-m-d', time() + DAY_IN_SECONDS ) );
// If setting timezone when saving data, $midnight += get_option( 'gmt_offset' ) * HOUR_IN_SECONDS;
$args = array(
    'post_type' => array( 'page' ),
    'order' => 'DESC',
    'orderby' => 'meta_value_num', // Numeric sort.
    'meta_query' => array(
        array(
            'key' => '_cmb_test_datetime_timestamp',
            'compare' => '<',
            'value' => $midnight,
            'type' => 'UNSIGNED', // Note was missing before.
        ),
    ),
    'numberposts' => 1,
);