_updateSecondTimestamp() {
      if (this.selectionMode === _CalendarSelectionMode.default.Range && (this.selectedDates.length === 1 || this.selectedDates.length === 2)) {
        this._secondTimestamp = this.timestamp;
      }
    }
During range selection, when the user is navigating with the keyboard,
the currently focused day is considered the "second day".
@private