code: public static TimestampBound ofMaxStaleness(long num, TimeUnit units) {
      checkStaleness(num);
      return new TimestampBound(Mode.MAX_STALENESS, null, createDuration(num, units));
    }
docs: Returns a timestamp bound that will perform reads and queries at a timestamp chosen to be at
  most {@code num units} stale. This 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 the freshest data available at a nearby replica, while bounding the
  possible staleness if the local replica has fallen behind.
  
  <p>Note that this option can only be used in single-use reads and single-use read-only
  transactions.