add_filter( 'woocommerce_get_price_html', 'custom_price_html', 100, 2 );
add_filter( 'woocommerce_cart_item_price', 'custom_price_html', 100, 2 );
function custom_price_html( $price, $product )
{
    global $post;
    $sales_price_to = get_post_meta($post->ID, '_sale_price_dates_to', true);
    $sales_price_from = get_post_meta($post->ID, '_sale_price_dates_from', true);

    if($sales_price_to != "")
    {
        setlocale(LC_TIME, 'pt_PT');
        $sales_price_date_to = strftime('%d %b', $sales_price_to);
        $sales_price_date_from = strftime('%d %b', $sales_price_from);

        return str_replace( '</ins>', ' </ins> <div class="promotion-text">De '.$sales_price_date_from.'. até '.$sales_price_date_to.'.</div>', $price );
    }
    else
    {
        return apply_filters( 'woocommerce_get_price', $price );
    }
}