// last_modified
        final Date lastModified = responseData.getLastModified();
        if (lastModified != null) {
            putResultDataBody(dataMap, fessConfig.getIndexFieldLastModified(), lastModified);
            // timestamp
            putResultDataBody(dataMap, fessConfig.getIndexFieldTimestamp(), lastModified);
        } else {
            // timestamp

     * comment: Field name for last modified date in the index.
     * @return The value of found property. (NotNull: if not found, exception but basically no way)
     */
    String getIndexFieldLastModified();

    /**
     * Get the value for the key 'index.field.anchor'. <br>
     * The value is, e.g. anchor <br>
     * comment: Field name for anchor in the index.