docs: double getFraction(Timestamp now) {
      Duration elapsedTime = now.subtractTimestamp(start);
      checkArgument(
          elapsedTime.compareTo(ZERO) >= 0 && elapsedTime.compareTo(duration) < 0,
          "This bucket must be current.");
      return ((double) elapsedTime.toMillis()) / duration.toMillis();
    }