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 is in.

edit(2): This is how I get the week number:

CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(DateTime.Now, CalendarWeekRule.FirstDay, DayOfWeek.Monday);

I'm just trying to do the reverse.

Answer 1

Wouldn't it also depend on the day of the week?

Answer 2

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 3

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

Answer 4

I think you're assuming that a "week" is any group of 7 sequential days. It isn't. Given Year(2008), Week(5), you could be in either January or Febuary, depending on when your "week" starts.

Answer 5

Another problem you could face is that most years do not start at the beginning of a week, which shifts everything.

Answer 6

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.

Answer 7

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 :)