Function add

packages/@internationalized/date/src/manipulation.ts:47–108

(
  date: CalendarDate | CalendarDateTime,
  duration: DateTimeDuration
)

  duration: DateTimeDuration
): CalendarDate | CalendarDateTime;
export function add(
  date: CalendarDate | CalendarDateTime,
  duration: DateTimeDuration
): Mutable<AnyCalendarDate | AnyDateTime> {
  let mutableDate: Mutable<AnyCalendarDate | AnyDateTime> = date.copy();
  let days = 'hour' in mutableDate ? addTimeFields(mutableDate, duration) : 0;

  addYears(mutableDate, duration.years || 0);
  if (mutableDate.calendar.balanceYearMonth) {
    mutableDate.calendar.balanceYearMonth(mutableDate, date);
  }

  mutableDate.month += duration.months || 0;

  balanceYearMonth(mutableDate);
  constrainMonthDay(mutableDate);

  mutableDate.day += (duration.weeks || 0) * 7;
  mutableDate.day += duration.days || 0;
  mutableDate.day += days;

  balanceDay(mutableDate);

  if (mutableDate.calendar.balanceDate) {
    mutableDate.calendar.balanceDate(mutableDate);
  }

  // Constrain in case adding ended up with a date outside the valid range for the calendar system.
  // The behavior here is slightly different than when constraining in the `set` function in that
  // we adjust smaller fields to their minimum/maximum values rather than constraining each field
  // individually. This matches the general behavior of `add` vs `set` regarding how fields are balanced.
  if (mutableDate.year < 1) {
    mutableDate.year = 1;
    mutableDate.month = 1;
    mutableDate.day = 1;
  }

  let maxYear = mutableDate.calendar.getYearsInEra(mutableDate);
  if (mutableDate.year > maxYear) {
    let isInverseEra = mutableDate.calendar.isInverseEra?.(mutableDate);
    mutableDate.year = maxYear;
    mutableDate.month = isInverseEra ? 1 : mutableDate.calendar.getMonthsInYear(mutableDate);
    mutableDate.day = isInverseEra ? 1 : mutableDate.calendar.getDaysInMonth(mutableDate);
  }

  if (mutableDate.month < 1) {
    mutableDate.month = 1;
    mutableDate.day = 1;
  }

  let maxMonth = mutableDate.calendar.getMonthsInYear(mutableDate);
  if (mutableDate.month > maxMonth) {
    mutableDate.month = maxMonth;
    mutableDate.day = mutableDate.calendar.getDaysInMonth(mutableDate);
  }

  mutableDate.day = Math.max(
    1,