Function addDays

core/src/components/datetime/utils/manipulation.ts:189–223

(refParts: DatetimeParts, numDays: number)

 * Currently can only go forward at most 1 month.
 */
export const addDays = (refParts: DatetimeParts, numDays: number) => {
  const { month, day, year } = refParts;
  if (day === null) {
    throw new Error('No day provided');
  }

  const workingParts = {
    month,
    day,
    year,
  };

  const daysInMonth = getNumDaysInMonth(month, year);
  workingParts.day = day + numDays;

  /**
   * If wrapping to next month
   * update days and increment month
   */
  if (workingParts.day > daysInMonth) {
    workingParts.day -= daysInMonth;
    workingParts.month += 1;
  }

  /**
   * If moving to next year, reset
   * month to January and increment year
   */
  if (workingParts.month > 12) {
    workingParts.month = 1;
    workingParts.year += 1;
  }

  return workingParts;
};

/**
 * Given DatetimeParts, generate the previous month.