_multipleSelection(timestamp) {
      const min = Math.min(...this.selectedDates);
      const max = Math.max(...this.selectedDates);
      let start;
      let end;
      let toggle = false;
      if (timestamp < min) {
        start = timestamp;
        end = min;
      } else if (timestamp >= min && timestamp <= max) {
        // inside the current range - toggle all between the selected and focused
        const distanceToMin = Math.abs(timestamp - min);
        const distanceToMax = Math.abs(timestamp - max);
        if (distanceToMin < distanceToMax) {
          start = timestamp;
          end = max;
        } else {
          start = min;
          end = timestamp;
        }
        toggle = true;
      } else {
        start = max;
        end = timestamp;
      }
      const startDate = _CalendarDate.default.fromTimestamp(start * 1000);
      const endDate = _CalendarDate.default.fromTimestamp(end * 1000);
      while (startDate.valueOf() <= endDate.valueOf()) {
        this[toggle ? "_toggleTimestampInSelection" : "_addTimestampToSelection"](startDate.valueOf() / 1000);
        startDate.setDate(startDate.getDate() + 1);
      }
    }
Called when at least one day is selected and the user presses "Shift".
@param { number } timestamp
@private