code: public static TimestampBound ofExactStaleness(long num, TimeUnit units) {
      checkStaleness(num);
      return new TimestampBound(Mode.EXACT_STALENESS, null, createDuration(num, units));
    }
docs: Returns a timestamp bound that will perform reads and queries at an exact staleness. The
  timestamp is chosen soon after the read is started.
  
  <p>Guarantees that all writes that have committed more than the specified number of seconds ago
  are visible. Because Cloud Spanner chooses the exact timestamp, this mode works even if the
  client's local clock is substantially skewed from Cloud Spanner commit timestamps.
  
  <p>Useful for reading at nearby replicas without the distributed timestamp negotiation overhead
  of {@link #ofMaxStaleness(long, TimeUnit)}.