getWeekNumber static method

int getWeekNumber(DateTime date, { int? firstWeekday, })

Returns week number in year.

The first week of the year is the week that contains 4 or more days of that year (='First 4-day week').

So if week contains less than 4 days - it's in another year.

The highest week number in a year is either 52 or 53.

You can define first weekday (Monday, Sunday or Saturday) with parameter firstWeekday. It should be one of the constant values DateTime.monday, ..., DateTime.sunday.

By default it's DateTime.monday.

Implementation

static int getWeekNumber(DateTime date, {int? firstWeekday}) {
  assert(firstWeekday == null || firstWeekday > 0 && firstWeekday < 8);

  if (isWeekInYear(date, date.year, firstWeekday)) {
    final startOfTheFirstWeek =
        firstDayOfFirstWeek(date.year, firstWeekday: firstWeekday);
    final diffInDays = getDaysDifference(date, startOfTheFirstWeek);
    return (diffInDays / DateTime.daysPerWeek).floor() + 1;
  } else if (date.month == DateTime.december) {
    // first of the next year
    return 1;
  } else {
    // last of the previous year
    return getWeekNumber(DateTime(date.year - 1, DateTime.december, 31),
        firstWeekday: firstWeekday);
  }
}