@Override
    public long endMillis() {
      try {
        return LongMath.checkedMultiply(windowMillis, (divPosition + 1));
      } catch (ArithmeticException ae) {
        return Long.MAX_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