Relevanssi Custom Meta Ordering of Search Results

/**
 * Adds a custom field to all posts, with the unixtime of a custom date OR the published date
 *
 * @param int $postId
 */
function customDateField($postId)
{
    // Generate my unix time
    $customDate = get_field('date');
    $searchDate = ($customDate) ? date('U', strtotime($customDate)) : get_post_time('U', TRUE);
    // You never need to add the post_meta, this will add it if needed
    update_post_meta($postId, 'searchdate', $searchDate);
    remove_filter('save_post', 'customDateField');
}

add_filter('save_post', 'customDateField');

/**
 * Allows the ordering by custom terms
 *
 * @param array $hits
 *
 * @return array mixed
 */
function customDateSearch($hits)
{
    if (get_query_var('orderby') == 'searchdate') {
        $dates = array();
        foreach ($hits[0] as $hit) {
            $searchdate = get_post_meta($hit->ID, 'searchdate', TRUE);
            if (!isset($dates[$searchdate])) {
                $dates[$searchdate] = array();
            }
            array_push($dates[$searchdate], $hit);
        }
        ksort($dates);
        $sorted_hits = array();
        foreach ($dates as $searchdate => $searchdate_hits) {
            $sorted_hits = array_merge($sorted_hits, $searchdate_hits);
        }
        // By default everything is sorted ascending, check if we need to change
        $hits[0] = ('asc' == strtolower(get_query_var('order'))) ? array_reverse($sorted_hits) : $sorted_hits;
    }
    return $hits;
}

add_filter('relevanssi_hits_filter', 'customDateSearch');