private long getMaxTierAgeCutoff(long now) {
    try {
      return LongMath.checkedSubtract(now, maxTierAgeMillis);
    } catch (ArithmeticException ae) {
      LOG.warn("Value for " + MAX_TIER_AGE_MILLIS_KEY + ": " + maxTierAgeMillis
        + ". Will always promote to next tier.");
      return Long.MIN_VALUE;
    }
  }
A timestamp t is within the window iff t / size == divPosition.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/compactions/ExponentialCompactionWindowFactory.java