Function addDuration

packages/graph/src/TemporalTypes.ts:1457–1622

(
  temporal: TemporalValue,
  duration: DurationValue,
)

 * Add a duration to a temporal value.
 */
export function addDuration(
  temporal: TemporalValue,
  duration: DurationValue,
): TemporalValue | null {
  if (temporal instanceof DateValue) {
    // Add months with proper month-end handling
    const afterMonths = addMonthsToDate(
      temporal.year,
      temporal.month,
      temporal.day,
      duration.months,
    );

    // Now add days
    const date = new Date(
      Date.UTC(afterMonths.year, afterMonths.month - 1, afterMonths.day + duration.days),
    );

    // Add time components (convert to days)
    const totalSeconds = duration.seconds + duration.nanoseconds / 1_000_000_000;
    const additionalDays = Math.floor(totalSeconds / 86400);
    date.setUTCDate(date.getUTCDate() + additionalDays);

    return new DateValue(date.getUTCFullYear(), date.getUTCMonth() + 1, date.getUTCDate());
  }

  if (temporal instanceof LocalTimeValue) {
    // Add duration to local time (only time components matter, wraps at 24h)
    let totalNanos =
      temporal.hour * 3_600_000_000_000 +
      temporal.minute * 60_000_000_000 +
      temporal.second * 1_000_000_000 +
      temporal.nanosecond +
      duration.seconds * 1_000_000_000 +
      duration.nanoseconds;

    // Handle days overflow (wrap around 24 hours)
    const nanosPerDay = 86_400_000_000_000;
    totalNanos = ((totalNanos % nanosPerDay) + nanosPerDay) % nanosPerDay;

    const hour = Math.floor(totalNanos / 3_600_000_000_000);
    const remainingAfterHours = totalNanos % 3_600_000_000_000;
    const minute = Math.floor(remainingAfterHours / 60_000_000_000);
    const remainingAfterMinutes = remainingAfterHours % 60_000_000_000;
    const second = Math.floor(remainingAfterMinutes / 1_000_000_000);
    const nanosecond = Math.round(remainingAfterMinutes % 1_000_000_000);

    return new LocalTimeValue(hour, minute, second, nanosecond);
  }

  if (temporal instanceof TimeValue) {
    // Add duration to time with offset
    let totalNanos =
      temporal.hour * 3_600_000_000_000 +
      temporal.minute * 60_000_000_000 +
      temporal.second * 1_000_000_000 +
      temporal.nanosecond +
      duration.seconds * 1_000_000_000 +