@Override
    public Window nextEarlierWindow() {
      // Don't promote to the next tier if there is not even 1 window at current tier
      // or if the next window crosses the max age.
      if (
        divPosition % windowsPerTier > 0
          || startMillis() - windowMillis * windowsPerTier < maxTierAgeCutoff
      ) {
        return new Window(windowMillis, divPosition - 1, maxTierAgeCutoff);
      } else {
        return new Window(windowMillis * windowsPerTier, divPosition / windowsPerTier - 1,
          maxTierAgeCutoff);
      }
    }
A timestamp t is within the window iff t / size == divPosition.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/compactions/ExponentialCompactionWindowFactory.java