Get the Correct Week Number of a Given Date

Get the correct week number of a given date

As noted in this MSDN page there is a slight difference between ISO8601 week and .Net week numbering.

You can refer to this article in MSDN Blog for a better explanation: "ISO 8601 Week of Year format in Microsoft .Net"

Simply put, .Net allow weeks to be split across years while the ISO standard does not.
In the article there is also a simple function to get the correct ISO 8601 week number for the last week of the year.

Update The following method actually returns 1 for 2012-12-31 which is correct in ISO 8601 (e.g. Germany).

// This presumes that weeks start with Monday.
// Week 1 is the 1st week of the year with a Thursday in it.
public static int GetIso8601WeekOfYear(DateTime time)
{
    // Seriously cheat.  If its Monday, Tuesday or Wednesday, then it'll 
    // be the same week# as whatever Thursday, Friday or Saturday are,
    // and we always get those right
    DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
    if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
    {
        time = time.AddDays(3);
    }

    // Return the week of our adjusted day
    return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
} 

How can I calculate/find the week-number of a given date?

var currentCulture = CultureInfo.CurrentCulture;
var weekNo = currentCulture.Calendar.GetWeekOfYear(
                new DateTime(2013, 12, 31),
                currentCulture.DateTimeFormat.CalendarWeekRule,
                currentCulture.DateTimeFormat.FirstDayOfWeek);

Be aware that this is not ISO 8601 compatible. In Sweden we use ISO 8601 week numbers but even though the culture is set to "sv-SE", CalendarWeekRule is FirstFourDayWeek, and FirstDayOfWeek is Monday the weekNo variable will be set to 53 instead of the correct 1 in the above code.

I have only tried this with Swedish settings but I'm pretty sure that all countries (Austria, Germany, Switzerland and more) using ISO 8601 week numbers will be affected by this problem.

Peter van Ooijen and Shawn Steele has different solutions to this problem.

Here's a compact solution

private static int WeekOfYearISO8601(DateTime date)
{
    var day = (int)CultureInfo.CurrentCulture.Calendar.GetDayOfWeek(date);
    return CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(date.AddDays(4 - (day == 0 ? 7 : day)), CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}

It's been tested for the following dates

var datesAndISO8601Weeks = new Dictionary<DateTime, int>
                        {
                            {new DateTime(2000, 12, 31), 52},
                            {new DateTime(2001, 1, 1), 1},
                            {new DateTime(2005, 1, 1), 53},
                            {new DateTime(2007, 12, 31), 1},
                            {new DateTime(2008, 12, 29), 1},
                            {new DateTime(2010, 1, 3), 53},
                            {new DateTime(2011, 12, 31), 52},
                            {new DateTime(2012, 1, 1), 52},
                            {new DateTime(2013, 1, 2), 1},
                            {new DateTime(2013, 12, 31), 1},
                        };

foreach (var dateWeek in datesAndISO8601Weeks)
{
    Debug.Assert(WeekOfYearISO8601(dateWeek.Key) == dateWeek.Value, dateWeek.Key.ToShortDateString() + " should be week number " + dateWeek.Value + " but was " + WeekOfYearISO8601(dateWeek.Key));
}

Get Current Week Number (C#)

This is probably what you are looking for:

DateTime dt = new DateTime(2020, 12, 21);
Calendar cal = new CultureInfo("en-US").Calendar;
int week = cal.GetWeekOfYear(dt, CalendarWeekRule.FirstDay, DayOfWeek.Monday);
Console.WriteLine(week);

You can change the CalendarWeekRule parameter to change the definition of the first week of the year:

FirstDay means that first week of the year can have any length. For example if the first day of the year was Sunday, it will be counted as week and the following Monday will be counted as part of second week.

FirstFourDayWeek means that the first week will be counted only if it mainly in this year. For example if the first day of the year will be Thursday the week will be counted, but if the year starts with Friday, the first week won't be counted.

FirstFullWeek means that the first week that will be counted will be the first full week of the year.

Get Week number from a given date when week start on Sunday

CalendarWeekRule Enumeration: Defines different rules for determining the first week of the year.

Previously what you were using:

CalendarWeekRule.FirstFourDayWeek

which needed to be replaced with:

CalendarWeekRule.FirstFullWeek

Because:

Code Snippet:

public static void Main()
{
    var week_test = Convert.ToDateTime("05/06/2018");
    var week_test2 = Convert.ToDateTime("05/13/2018");
    List<DateTime> weekList = new List<DateTime>();
    weekList.Add(week_test);
    weekList.Add(week_test2);
    CultureInfo ciCurr = CultureInfo.CurrentCulture;
  foreach(var week in weekList)
  {
    int weekNum = ciCurr.Calendar.GetWeekOfYear(week, CalendarWeekRule.FirstFullWeek, DayOfWeek.Sunday);
    Console.WriteLine(weekNum);
  }
}

Output:

Demo:

dotNetFiddle

Get the week number from a date time

There's a blog article explaining this behavior and proposing a solution.

The issue:

Several people have noticed that Calendar.GetWeekOfYear() is almost like the ISO 8601 week when passed CalendarWeekRule.FirstFourDayWeek and DayOfWeek.Monday, however it is a little bit different. Specifically ISO 8601 always has 7 day weeks. If the first partial week of a year doesn't contain Thursday, then it is counted as the last week of the previous year. Likewise, if the last week of the previous year doesn't contain Thursday then its treated like the first week of the next year. GetWeekOfYear() has the first behavior, but not the second.

The proposed solution would be this:

A simple workaround to consistently get the ISO 8601 week is to realize that consecutive days Monday through Sunday in ISO 8601 weeks all have the same week #. So Monday has the same week # as Thursday. Since Thursday is the critical day for determining when the week starts each year my solution is to add 3 days if the day is Monday, Tuesday or Wednesday. The adjusted days are still in the same week, and use values that GetWeekOfYear and ISO 8601 agree on.

// This presumes that weeks start with Monday.
// Week 1 is the 1st week of the year with a Thursday in it.
public static int GetIso8601WeekOfYear(DateTime time)
{
    // Seriously cheat.  If its Monday, Tuesday or Wednesday, then it'll 
    // be the same week# as whatever Thursday, Friday or Saturday are,
    // and we always get those right
    DayOfWeek day = cal.GetDayOfWeek(time);
    if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
    {
        time = time.AddDays(3);
    }

    // Return the week of our adjusted day
    return cal.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}

All credits for this go to Shawn Steele.

How do I calculate the week number given a date?

Pseudocode:

int julian = getDayOfYear(myDate)  // Jan 1 = 1, Jan 2 = 2, etc...
int dow = getDayOfWeek(myDate)     // Sun = 0, Mon = 1, etc...
int dowJan1 = getDayOfWeek("1/1/" + thisYear)   // find out first of year's day
// int badWeekNum = (julian / 7) + 1  // Get our week# (wrong!  Don't use this)
int weekNum = ((julian + 6) / 7)   // probably better.  CHECK THIS LINE. (See comments.)
if (dow < dowJan1)                 // adjust for being after Saturday of week #1
    ++weekNum;
return (weekNum)

To clarify, this algorithm assumes you number your weeks like this:

S  M  T  W  R  F  S
            1  2  3    <-- week #1
4  5  6  7  8  9 10    <-- week #2
[etc.]

getDayOfWeek() and getDayOfYear() are standard date-object operations in most languages. If yours doesn't have them, you can count-forward from some known date (Jan 1, 1970 is a common one), after looking up to see what day of the week it was.

If you're going to implement your own date counting routines, remember that years that are divisible by 100 are NOT leap years, unless they are also divisible by 400. So 1900 was not a leap year, but 2000 was. If you're going to work far back in time, you have to mess with Gregorian vs Julian calendars, etc., see Wikipedia for loads of info on that.

This link talks about date/time functions in Windows/C++ in greater detail.

Get the correct week number of a given date

As noted in this MSDN page there is a slight difference between ISO8601 week and .Net week numbering.

You can refer to this article in MSDN Blog for a better explanation: "ISO 8601 Week of Year format in Microsoft .Net"

Simply put, .Net allow weeks to be split across years while the ISO standard does not.
In the article there is also a simple function to get the correct ISO 8601 week number for the last week of the year.

Update The following method actually returns 1 for 2012-12-31 which is correct in ISO 8601 (e.g. Germany).

// This presumes that weeks start with Monday.
// Week 1 is the 1st week of the year with a Thursday in it.
public static int GetIso8601WeekOfYear(DateTime time)
{
    // Seriously cheat.  If its Monday, Tuesday or Wednesday, then it'll 
    // be the same week# as whatever Thursday, Friday or Saturday are,
    // and we always get those right
    DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);
    if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
    {
        time = time.AddDays(3);
    }

    // Return the week of our adjusted day
    return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
} 

Read the correct week number from a calendar date

Don't create huge procedures. Small is beautiful. Create functions that feed into your Main procedure. Here is an example. The procedure TestExtraction calls the function ExtractWeeks. Therefore ExtractWeeks needs not be part of the procedure that calls it, making the code easier to understand and maintain.

Private Sub TestExtraction()

    Dim Fun As Long
    Dim DateString As String
    Dim StartDate As Date, EndDate As Date

    DateString = ActiveCell.Value

    ' the DateString is re-defined here for testing purposes
    DateString = "[10.04.2017 - 05.05.2017]"


    Fun = ExtractWeeks(DateString, StartDate, EndDate)
    If Fun < 0 Then
        Debug.Print "Invalid date"
    Else
        With Application
            DateString = "(CW" & .WeekNum(StartDate)
            If Year(StartDate) <> Year(EndDate) Then _
                DateString = DateString & "/" & Year(StartDate)
            DateString = DateString & " - " & .WeekNum(EndDate) & "/" & Year(EndDate) & ")"
        End With
        Debug.Print DateString
        Debug.Print Fun & " weeks"
    End If
End Sub

Private Function ExtractWeeks(ByVal DateString As String, _
                              StartDate As Date, _
                              EndDate As Date) As Long
    ' 24 Oct 2017
    ' return the number of weeks between dates (rounded up)
    ' return -1 if one of the dates is unreadable

    Dim Dates() As String
    Dim i As Integer

    Dates = Split(Mid(DateString, 2, Len(DateString) - 2), "-")
    On Error Resume Next
    For i = 0 To 1
        Dates(i) = Replace(Trim(Dates(i)), ".", Application.International(xlDateSeparator))
    Next i
    StartDate = DateValue(Dates(0))
    EndDate = DateValue(Dates(1))
    If Err Then
        ExtractWeeks = -1
    Else
        ExtractWeeks = Int((StartDate - EndDate) / 7) * -1
    End If
End Function

The point is that not everything that looks like a date is a date Excel can understand. The Function ExtractWeeks converts the "dates' from your worksheet into real dates and returns these dates to the calling procedure. It also returns -1 in case of error which you can use to trap such errors. In my example, the function returns the number of weeks (or -1). You might let it return the CW string my calling procedure constructs. You will find it easy to move the process of constructing that string to the function and let the function return "" in case of error instead of -1. Perhaps you can exclude the possibility of errors in the dates. This is a question of how you integrate the function into your Main.

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