Set the_date() according to custom field = yyyy-mm-dd 12:45

function change_post_date( $post_id, $post ) {

    // WordPress calls "save_post" when a post is saved, updated, autosaved,
    // revision created, or ajax called. We only want to execute this function
    // during post save and update. The next 3 "if" statements check to see why
    // save_post was called...

    // Autosave? Do nothing
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
        return;
    // AJAX? Do nothing
    if ( defined( 'DOING_AJAX' ) && DOING_AJAX )
        return;
    // Post revision? Do nothing
    if ( false !== wp_is_post_revision( $post_id ) )
        return;

    // Make sure the person editing the post has permission to do so
    if ( ! current_user_can( 'edit_post', $post_id ) )
        return;

    // Get the "date-time" field
    $date_time_field = get_post_meta($post_id, 'date-time', true);

    // Unhook
    remove_action('save_post', 'change_post_date', 10);

    $args = array (
        'ID' => $post_id,
        'post_date' => $date_time_field,
        'post_date_gmt' => gmdate('Y-m-d H:i:s', strtotime($date_time_field) )
    );

    wp_update_post( $args );

    // Re-hook
    add_action('save_post', 'change_post_date', 10);

}
add_filter('save_post', 'change_post_date', 10, 2);