code: function isSelected(selectedDates, date) {
    selectedDates = Array.isArray(selectedDates)
      ? selectedDates
      : [selectedDates];
    return selectedDates.some(selectedDate => {
      if (
        selectedDate instanceof Date &&
        startOfDay(selectedDate).getTime() === startOfDay(date).getTime()
      ) {
        return true;
      }
      return false;
    });
  }
docs: Normalizes dates to the beginning of the day,
  then checks to see if the day given is found
  in the selectedDates.
  @param {Array.<Date>} selectedDates An array of dates currently selected
  @param {Date} date The date to search with against selectedDates
  @returns {Boolean} Whether day is found in selectedDates