Bug in WeekNumber calculation .NET?

Question

I have a rather weird problem. I live in denmark and here the first week (Week 1) of 2013 starts the 31th of december 2012 and lasts for 7 days - as weeks normally do :)

According to .NET however the 30th of december is Week 52, the 31th is Week 53 and the 1st of January is Week 1.

Week 53 lasts for only one day, and Week 1 for 6 days. Clearly this must be wrong (a week consisting of less than 7 days) and certainly is wrong in danish context. Where the 31th of december is Week 1, NOT Week 53.

The following code illustrates the problem (CurrentCulture is "da-DK")

    static void Main(string[] args)
    {
        //Here I get Monday
        DayOfWeek firstDayOfWeek = DateTimeFormatInfo.CurrentInfo.FirstDayOfWeek;             
        //Here I get FirstFourDayWeek
        CalendarWeekRule weekRule = DateTimeFormatInfo.CurrentInfo.CalendarWeekRule; 

        DateTime date = new DateTime(2012,12,30);

        for (int i = 0; i <= 10; i++)
        {
            DateTime currentDate = date.AddDays(i);
            Console.WriteLine("Date: {0} WeekNumber: {1}",
                currentDate.ToShortDateString(),
                CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(currentDate, weekRule, firstDayOfWeek));
        }
        Console.ReadLine();
    }

Have I done something wrong or is this a bug in .NET ? If the latter - do you have suggestions for calculating weeknumbers correctly ?

Answer 1

The problem is that the GetWeekOfYear method does not respect ISO 8601, which is what you expect, but it doesn't.

Note that while you are using FirstFourDayWeek, the documentation says:

The first week based on the FirstFourDayWeek value can have four to seven days.

which is a violation of the ISO 8601 rule that all weeks have to have seven days.

Also:

---

You can use the following method to obtain the correct week number according to ISO 8601:

int weekNumber(DateTime fromDate)
{
    // Get jan 1st of the year
    DateTime startOfYear = fromDate.AddDays(- fromDate.Day + 1).AddMonths(- fromDate.Month +1);
    // Get dec 31st of the year
    DateTime endOfYear = startOfYear.AddYears(1).AddDays(-1);
    // ISO 8601 weeks start with Monday 
    // The first week of a year includes the first Thursday 
    // DayOfWeek returns 0 for sunday up to 6 for saterday
    int[] iso8601Correction = {6,7,8,9,10,4,5};
    int nds = fromDate.Subtract(startOfYear).Days  + iso8601Correction[(int)startOfYear.DayOfWeek];
    int wk = nds / 7;
    switch(wk)
    {
        case 0 : 
            // Return weeknumber of dec 31st of the previous year
            return weekNumber(startOfYear.AddDays(-1));
        case 53 : 
            // If dec 31st falls before thursday it is week 01 of next year
            if (endOfYear.DayOfWeek < DayOfWeek.Thursday)
                return 1;
            else
                return wk;
        default : return wk;
    }
}

Source (there are also plenty other functions out there...)

---

So, changing your loop to

for (int i = 0; i <= 10; i++)
{
    DateTime currentDate = date.AddDays(i);
    Console.WriteLine("Date: {0} WeekNumber: {1}: CorrectWeekNumber: {2}",
        currentDate.ToShortDateString(),
        CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(currentDate, weekRule, firstDayOfWeek),
        weekNumber(currentDate));
}

will result in:

Date: 30.12.2012 WeekNumber: 52: CorrectWeekNumber: 52
Date: 31.12.2012 WeekNumber: 53: CorrectWeekNumber: 1
Date: 01.01.2013 WeekNumber: 1: CorrectWeekNumber: 1
Date: 02.01.2013 WeekNumber: 1: CorrectWeekNumber: 1
Date: 03.01.2013 WeekNumber: 1: CorrectWeekNumber: 1
Date: 04.01.2013 WeekNumber: 1: CorrectWeekNumber: 1
Date: 05.01.2013 WeekNumber: 1: CorrectWeekNumber: 1
Date: 06.01.2013 WeekNumber: 1: CorrectWeekNumber: 1
Date: 07.01.2013 WeekNumber: 2: CorrectWeekNumber: 2
Date: 08.01.2013 WeekNumber: 2: CorrectWeekNumber: 2
Date: 09.01.2013 WeekNumber: 2: CorrectWeekNumber: 2