getNthWeekdayOfMonthInYear method

DateTime? getNthWeekdayOfMonthInYear(int n, int dayOfWeek)

Returns the date of the nth occurrence of a specific weekday in a given month and year.

Args: n (int): The desired occurrence (e.g., 1 for the 1st, 2 for the 2nd, etc.). dayOfWeek (int): The day of the week (e.g., DateTime.monday, DateTime.tuesday, etc.). month (int): The month (1-12). year (int): The year.

Returns: DateTime?: The date of the nth occurrence, or null if it does not exist (e.g., 5th Friday in February).

Implementation

DateTime? getNthWeekdayOfMonthInYear(int n, int dayOfWeek) {
  if (n < 1) {
    return null; // Invalid input: n must be at least 1
  }

  // Get the first day of the month for the current year
  final DateTime firstDayOfMonth = DateTime(year, month);

  // Calculate the offset to the first occurrence of the desired day
  // of the week
  final int offset = (dayOfWeek - firstDayOfMonth.weekday + 7) % 7;

  // Calculate the date of the first occurrence
  final DateTime firstOccurrence = firstDayOfMonth.add(Duration(days: offset));

  // Calculate the date of the nth occurrence
  final DateTime nthOccurrence = firstOccurrence.add(Duration(days: (n - 1) * 7));
  // Check if the nth occurrence is within the target month
  if (nthOccurrence.month != month) {
    return null; // The nth occurrence falls outside the month
  }

  // **Set time components to zero (midnight)**
  return DateTime(nthOccurrence.year, nthOccurrence.month, nthOccurrence.day);
}