@Override
    public long startMillis() {
      try {
        return LongMath.checkedMultiply(windowMillis, divPosition);
      } catch (ArithmeticException ae) {
        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