code: function nextMonth() {
    if (calendar.m === 12) {
      calendar.m = 1;
      calendar.y += 1;
      calendar.d = 1;
    } else {
      calendar.m += 1;
      calendar.d = 1;
    }
  
    calendar.year = yearFactory.get(calendar.y);
  }
docs: Goes to the next month.
  @private