cal.pub0.org/packages/features/calendars/lib/getAvailableDatesInMonth.ts

import { daysInMonth, yyyymmdd } from "@calcom/lib/date-fns";

// calculate the available dates in the month:
// *) Intersect with included dates.
// *) Dates in the past are not available.
// *) Use right amount of days in given month. (28, 30, 31)
export function getAvailableDatesInMonth({
  browsingDate,
  minDate = new Date(),
  includedDates,
}: {
  browsingDate: Date;
  minDate?: Date;
  includedDates?: string[];
}) {
  const dates = [];
  const lastDateOfMonth = new Date(
    browsingDate.getFullYear(),
    browsingDate.getMonth(),
    daysInMonth(browsingDate)
  );
  for (
    let date = browsingDate > minDate ? browsingDate : minDate;
    date <= lastDateOfMonth;
    date = new Date(date.getFullYear(), date.getMonth(), date.getDate() + 1)
  ) {
    // intersect included dates
    if (includedDates && !includedDates.includes(yyyymmdd(date))) {
      continue;
    }
    dates.push(yyyymmdd(date));
  }
  return dates;
}