code: public static void checkIfUnmodifiedSince(final String ifUnmodifiedSince, final Instant modified) {
          final Instant time = parseDate(ifUnmodifiedSince);
          if (time != null && modified.truncatedTo(SECONDS).isAfter(time)) {
              throw new ClientErrorException(status(PRECONDITION_FAILED).build());
          }
      }
docs: Check for a conditional operation.
  @param ifUnmodifiedSince the If-Unmodified-Since header
  @param modified the resource modification date