How do I get the month number from the year and week number in c#?

Question

As the title says, given the year and the week number, how do I get the month number?

edit: if a week crosses two months, I want the month the first day of the week

Answer 1

You cant. You need at least the day on which the 1st week starts (or when the week starts), to get an accurate answer.

Answer 2

This is what I ended up doing:

static int GetMonth(int Year, int Week)
{
    DateTime tDt = new DateTime(Year, 1, 1);

    tDt.AddDays((Week - 1) * 7);

    for (int i = 0; i <= 365; ++i)
    {
        int tWeek = CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(
            tDt, 
            CalendarWeekRule.FirstDay, 
            DayOfWeek.Monday);
        if (tWeek == Week)
            return tDt.Month;

        tDt = tDt.AddDays(1);
    }
    return 0;
}

I would have preferred something simpler, but it works :)

Answer 3

// Calculate the week number according to ISO 8601

    public static int Iso8601WeekNumber(DateTime dt)
    {
            return  CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(dt, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
    }

// ...

DateTime dt = DateTime.Now;

// Calculate the WeekOfMonth according to ISO 8601
int weekOfMonth = Iso8601WeekNumber(dt) - Iso8601WeekNumber(dt.AddDays(1 - dt.Day)) + 1;

Answer 4

You cant. A week may start in one month and end in another.

Answer 5

If you assume that the first day of your definition of week is the same day as the 1st day of the year, then this will work:

int year = 2000;
int week = 9;
int month = new DateTime(year, 1, 1).AddDays(7 * (week - 1)).Month;

Obviously, a true answer would depend on how you define the first day of the week, and how you define how a week falls into a month when it overlaps more than one.