code: private long parseLastModified(HttpResponse response) {
          Header header = response.getLastHeader("Last-Modified");
          if (header == null) {
              return 0;
          }
          String value = header.getValue();
          if (value == null || value.isEmpty()) {
              return 0;
          }
          Date date = DateUtils.parseDate(value);
          if (date == null) {
              return 0;
          }
          return date.getTime();
      }
docs: Parse the Last-Modified header of a {@link HttpResponse}
  @param response the {@link HttpResponse}
  @return the last-modified value or 0 if it is unknown