Vanilla JavaScript (ES6) Class that calculates monthly calendar object

Calendar.js

export default class Calendar {
  constructor(...args) {
    this.date = new Date(...args);
  }

  get year() {
    return this.date.getFullYear();
  }

  get month() {
    return this.monthIndex + 1;
  }

  get monthIndex() {
    return this.date.getMonth();
  }

  beginningOfMonth(diff = 0) {
    // NOTE: it's safe to add diff to monthIndex even if it's Jan or Dec.
    return new Date(this.year, this.monthIndex + diff);
  }

  get nextMonth() {
    return this.beginningOfMonth(1);
  }

  get lastMonth() {
    return this.beginningOfMonth(-1);
  }

  get next() {
    return new Calendar(this.nextMonth);
  }

  get prev() {
    return new Calendar(this.lastMonth);
  }

  get firstDay() {
    return this.beginningOfMonth().getDay();
  }

  get daysInMonth() {
    return 32 - new Date(this.year, this.monthIndex, 32).getDate();
  }

  get weeks() {
    const { year, monthIndex, firstDay, daysInMonth } = this;
    const weeks = [];
    let date = 1;

    for (let week = 0; week < 6; week++) {
      if (date > daysInMonth) { break; }
      const days = [];

      for (let day = 0; day < 7; day++) {
        if (week === 0 && day < firstDay) {
          days.push(new Date(year, monthIndex, day - firstDay + 1));
        } else {
          days.push(new Date(year, monthIndex, date++));
        }
      }

      weeks.push(days);
    }

    return weeks;
  }

  eq(other) {
    return (this - other) === 0;
  }

  valueOf() {
    return this.beginningOfMonth().valueOf();
  }

  toString() {
    let { year, month } = this;
    month = `0${month}`.substr(-2);
    return `${year}-${month}-01`;
  }

  includes(date) {
    return date.getFullYear() == this.year && date.getMonth() == this.monthIndex;
  }
}