Bug in WeekNumber calculation .NET?

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 ?

Answers


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


Thanks for all the answers. I also searched some more and finally created two C# methods to achieve what I wanted:

First a concise one found in one of the comments at: http://blogs.msdn.com/b/shawnste/archive/2006/01/24/iso-8601-week-of-year-format-in-microsoft-net.aspx

Which Jon Senchyna also pointed to:

     public static int WeekNumber(this DateTime date)
    {
        Calendar cal = CultureInfo.InvariantCulture.Calendar;
        DayOfWeek day = cal.GetDayOfWeek(date);
        date = date.AddDays(4 - ((int)day == 0 ? 7 : (int)day));
        return cal.GetWeekOfYear(date, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
    }

And also one at: http://www.tondering.dk/claus/cal/week.php#calcweekno

    public static int WeekNumber2(this DateTime date)
    {
        int a;
        int b;
        int c;
        int s;
        int e;
        int f;

        if (date.Month <= 2)
        {
            a = date.Year - 1;
            b = a / 4 - a / 100 + a / 400;
            c = (a - 1) / 4 - (a - 1) / 100 + (a - 1) / 400;
            s = b - c;
            e = 0;
            f = date.Day - 1 + 31 * (date.Month - 1);
        }
        else
        {
            a = date.Year;
            b = a / 4 - a / 100 + a / 400;
            c = (a - 1) / 4 - (a - 1) / 100 + (a - 1) / 400;
            s = b - c;
            e = s + 1;
            f = date.Day + ((153 * (date.Month - 3) + 2) / 5) + 58 + s;
        }

        int g = (a + b) % 7;
        int d = (f + g - e) % 7;
        int n = f + 3 - d;

        if (n < 0)
            return 53 - ((g - s) / 5);
        if (n > (364 + s))
            return 1;
        return n / 7 + 1;
    }

Both gave me what I wanted.

I also wrote a small unittest that proves that they return the same weeknumbers for the first 3000 years of the calendar.

    [TestMethod]
    public void WeekNumbers_CorrectFor_3000Years()
    {
        var weekNumbersMethod1 = WeekNumbers3000Years(DateManipulation.WeekNumber).ToList();
        var weekNumbersMethod2 = WeekNumbers3000Years(DateManipulation.WeekNumber2).ToList();
        CollectionAssert.AreEqual(weekNumbersMethod1, weekNumbersMethod2);
    }

    private IEnumerable<int> WeekNumbers3000Years(Func<DateTime, int> weekNumberCalculator)
    {
        var startDate = new DateTime(1,1,1);
        var endDate = new DateTime(3000, 12, 31);
        for(DateTime date = startDate; date < endDate; date = date.AddDays(1))
            yield return weekNumberCalculator(date);
    }

1.1.2013 is tuesday, and starts week #1. 31.12.2012 is monday, belongs to year 2012 and thus is week #53. Naturally, you can't have week 1 on December as that would mean there would two week 1's in a year, and thus would cause all sorts of nasty problems with code.

There is no rule that a week must be exactly 7 days long.


The Calendar.GetWeekOfYear doesn't' support the ISO8601 specification, see also ISO 8601 Week of Year format in Microsoft .Net.

You can use the Week class of the Time Period Library for .NET:

// ----------------------------------------------------------------------
public void CalendarWeekSample()
{
  DateTime testDate = new DateTime( 2007, 12, 31 );

  // .NET calendar week
  TimeCalendar calendar = new TimeCalendar();
  Console.WriteLine( "Calendar Week of {0}: {1}", testDate.ToShortDateString(),
                     new Week( testDate, calendar ).WeekOfYear );
  // > Calendar Week of 31.12.2007: 53

  // ISO 8601 calendar week
  TimeCalendar calendarIso8601 = new TimeCalendar(
    new TimeCalendarConfig { YearWeekType = YearWeekType.Iso8601 } );
  Console.WriteLine( "ISO 8601 Week of {0}: {1}", testDate.ToShortDateString(),
                     new Week( testDate, calendarIso8601 ).WeekOfYear );
  // > ISO 8601 Week of 31.12.2007: 1
} // CalendarWeekSample

I don't know anything about the Danish calendar, but the rules for .NET are clear, and your code is corresponding to those exactly. MSDN documentation for the CalendarWeekRule enumeration is here, for reference. In summary:

.NET will always treat 31 December as being in "the last week of the year."

You say you're getting a value of FirstFourDayWeek for the week rule. Since 1 Jan 2013 is a Tuesday, the remainder of that week has six days (the next week starts on the following Monday). Therefore, 1-6 Jan 2013 is considered "Week 1." If you had FirstFullWeek for that rule, week 1 would start on Monday 7th instead.

As for "Clearly this must be wrong:" clearly it's right, according to its own spec. There is never a requirement for any programming language or API to conform to a particular expectation; each project defines its own requirements. .NET is following rules that we may deem unexpected, but they are documented, and it's consistent to its documentation.

Whether that's useful or not is another question...


You can use the following code in order to calculate the week number for a given date:

    public static int GetWeekNumber(DateTime date)
    {
        var firstDayOfYear = new DateTime(date.Year, 1, 1);
        var lastDayOfYear = new DateTime(date.Year, 12, 31);
        var lastDayOfPreviousYear = new DateTime(date.Year - 1, 12, 31);

        var weekDayOfFirstDayOfYear = (int)firstDayOfYear.DayOfWeek + 1;
        var weekDayOfLastDayOfYear = (int)lastDayOfYear.DayOfWeek + 1;
        var days = (date - firstDayOfYear).Days;

        if (days <= 7 - weekDayOfFirstDayOfYear)  // My day fall in 1'st week of the year
        {
            if (weekDayOfFirstDayOfYear > 5)
                return GetWeekNumber(lastDayOfPreviousYear);
            return 1;
        }
        else // My day fall not on 1'st week of the year
        {
            // Number of weeks that pass from 1'st Sunday of 2'nd week of the year
            var weekNo = ((days - (8 - weekDayOfFirstDayOfYear)) / 7) + 1;

            if (weekDayOfFirstDayOfYear < 6)  // if Year start at Sun...Thursday the first week is added.
                weekNo++;

            // Check if Last week of the year belong to next year
            if (weekDayOfLastDayOfYear < 5) // if the year end in Sunday to Wednesday then it might belong to the 1'st week of the next year
            {
                if ((lastDayOfYear - date).Days < weekDayOfLastDayOfYear)
                {
                    return 1;
                }
            }

            return weekNo;
        }
    }

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