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

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

Answer 2

Assumptions:

1. Sunday is the first day of the week.
2. Partial week still counts as week 1

3. Outputs beginning and ending month as integer array.

   public int[] getMonth(int weekNum, int year)
   {
   
       DateTime StartYear = new DateTime(year, 1, 1);
       System.DayOfWeek StartDOW = StartYear.DayOfWeek;
       DateTime DayOfYearWeekStart = default(DateTime);
       DateTime DayOfYearWeekEnd = default(DateTime);
       int x = 0;
       if ((StartDOW == System.DayOfWeek.Sunday)) {
           DayOfYearWeekStart = StartYear.AddDays((weekNum - 1) * 7);
           DayOfYearWeekEnd = DayOfYearWeekStart.AddDays(6);
       } else {
           for (x = 0; x <= 7; x += 1) {
               if (StartYear.AddDays(x).DayOfWeek == DayOfWeek.Sunday) {
                   break; // TODO: might not be correct. Was : Exit For
               }
           }
   
           if (weekNum == 1) {
               DayOfYearWeekStart = StartYear;
               DayOfYearWeekEnd = StartYear.AddDays(x - 1);
           } else if (weekNum > 1) {
               DayOfYearWeekStart = StartYear.AddDays(((weekNum - 2) * 7) + x);
               DayOfYearWeekEnd = DayOfYearWeekStart.AddDays(6);
           }
   
       }
   
       int[] Month = new int[2];
       Month[0] = DayOfYearWeekStart.Month;
       Month[1] = DayOfYearWeekEnd.Month;
   
       return Month;
   }
   

Answer 3

this should be able to help

 public int getMonth(int weekNum, int year)
 {
     DateTime Current = new DateTime(year, 1, 1);
     System.DayOfWeek StartDOW = Current.DayOfWeek;
     int DayOfYear = (weekNum * 7) - 6; //1st day of the week

     if (StartDOW != System.DayOfWeek.Sunday) //means that last week of last year's month
     {
         Current = Current.AddDays(7 - (int)Current.DayOfWeek);
     }
     return Current.AddDays(DayOfYear).Month;
}

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

In .NET 3.0 and later you can use the ISOWeek-Class.

public static int MonthOfFirstDay(int year, int week)
{
    return ISOWeek.ToDateTime(year, week, DayOfWeek.Monday).Month;
}

Note that the year might not fit, as the first week of a year can already start in end of December the year before. For instance the first week of 2020 started on Monday 2019-12-30.