WooCommerce - Business hours.

functions.php

<?php

/**
 * Business hours.
 */
function odin_wc_business_hours_redirect() {
    if ( is_checkout() || is_cart() ) {
        global $woocommerce;

        // Set the timezone.
        date_default_timezone_set('America/Sao_Paulo');
        // Get the current hour.
        $hour = date( 'G' );

        if ( ! ( $hour >= 8 AND $hour <= 18 ) ) {
            $woocommerce->add_error( __( 'Fora do Horário de Funcionamento', 'odin' ) );
            wp_redirect( get_permalink( woocommerce_get_page_id( 'shop' ) ) );
            exit;
        }
    }
}

add_action( 'template_redirect', 'odin_wc_business_hours_redirect', 1 );