let d = Temporal.Duration.from({
  nanoseconds: 0,
});

let cal = new class extends Temporal.Calendar {
  dateAdd(date, duration, options) {
    if (duration.years === 1) {
      return date;
    }
    return super.dateAdd(date, duration, options);
  }
}("iso8601");

let date = new Temporal.PlainDate(1970, 1, 1, cal);

let r = d.round({
  relativeTo: date,
  smallestUnit: "years",
});