static void ensureDate(final HeaderGroup headers, final Instant instant) {
        if (!headers.containsHeader(HttpHeaders.DATE)) {
            headers.addHeader(new BasicHeader(HttpHeaders.DATE, DateUtils.formatStandardDate(instant)));
        }
    }
This method should be provided by the core
httpclient5-cache/src/main/java/org/apache/hc/client5/http/cache/HttpCacheEntryFactory.java