react-day-picker/src/helpers/getInitialMonth.ts

Customizable Date Picker for React

import { TZDate } from "@date-fns/tz";

import { type DateLib } from "../classes/DateLib.js";
import { type DayPickerProps } from "../types/props.js";

/** Return the start month based on the props passed to DayPicker. */
export function getInitialMonth(
  props: Pick<
    DayPickerProps,
    | "fromYear"
    | "toYear"
    | "startMonth"
    | "endMonth"
    | "month"
    | "defaultMonth"
    | "today"
    | "numberOfMonths"
    | "timeZone"
  >,
  dateLib: DateLib
): Date {
  const {
    month,
    defaultMonth,
    today = dateLib.today(),
    numberOfMonths = 1,
    endMonth,
    startMonth,
    timeZone
  } = props;
  let initialMonth = month || defaultMonth || today;
  const { differenceInCalendarMonths, addMonths, startOfMonth } = dateLib;

  // Fix the initialMonth if is after the endMonth
  if (endMonth && differenceInCalendarMonths(endMonth, initialMonth) < 0) {
    const offset = -1 * (numberOfMonths - 1);
    initialMonth = addMonths(endMonth, offset);
  }
  // Fix the initialMonth if is before the startMonth
  if (startMonth && differenceInCalendarMonths(initialMonth, startMonth) < 0) {
    initialMonth = startMonth;
  }
  // When timeZone is provided, convert initialMonth to TZDate type to ensure proper timezone handling
  initialMonth = timeZone ? new TZDate(initialMonth, timeZone) : initialMonth;

  return startOfMonth(initialMonth);
}