buildCalendarGrid.ts

const grid = [];
let week: CalendarGridDate[] = [];

const lastDay = new Date(year, month + 1, 0);
const firstDayInWeek = getDay(firstDay);
const daysInMonth = lastDay.getDate();

const daysOutsideMonthInLastWeek = 7 - getDay(lastDay);
const totalDaysInCalendar = daysInMonth + firstDayInWeek + daysOutsideMonthInLastWeek;

for (let i = 0; i < totalDaysInCalendar; i++) {
  const dayIndexInMonth = i - firstDayInWeek;
  week.push(createGridDate(addDays(firstDay, dayIndexInMonth), isOutsideMonth(dayIndexInMonth, daysInMonth)));
  if (week.length === 7) {
    grid.push(week);
    week = [];
  }
}