FacetWP – index date based on UNIX timestamp

<?php

function fwp_index_timestamp( $params, $class ) {
    if ( 'class_start_date_range' == $params['facet_name'] ) {
        $raw_value = $params['facet_value'];

        // basic timestamp validation
        if ( is_numeric( $raw_value ) ) {
            $ymd = date( 'Y-m-d', (int) $raw_value );
            $params['facet_value'] = $ymd;
            $params['facet_display_value'] = $ymd;
        }
    }
    return $params;
}

add_filter( 'facetwp_index_row', 'fwp_index_timestamp', 10, 2 );