VBA Convert date to week number

Question

In VBA I want to convert a date as 03/11/2017(DD/MM/YYYY) into the week number for that date.

Until now I have the following code:

   'geting the date out of the string
    HeadlineTemp = Mid(VRHeadline, InStr(VRHeadline, "[") + 1, 10)
   'switch "." to "/"
    HeadlineTemp = Replace(HeadlineTemp, ".", "/")
   'convert to a date
    FristVRFirstKW = CDate(HeadlineTemp)

Now, I need a function to convert that date into the week number of the year. First week day is Monday.

Answer 1

WeekdayName( number, [abbreviate], [firstdayofweek] ) WeekdayName(2) Result: 'Monday'

WeekdayName(2, TRUE) Result: 'Mon'

WeekdayName(2, TRUE, vbMonday) Result: 'Mon'

Answer 2

To make the Week Number with Monday as a first day, use the following:

WorksheetFunction.WeekNum(now, vbMonday)

Answer 3

Using VBA, to convert a date into an isoWeeknumber, you merely need the DatePart function (where DT is the date of interest):

isoWeekNumber = DatePart("ww", DT, vbMonday, vbFirstFourDays)

If you want to use other definitions than that specified in ISO 8601, investigate some of the other options for FirstDayOfWeek and FirstWeekOfYear

NOTE

As pointed out by @Mike85, there is a bug in DatePart (and also in the Format) function wherein Monday may be erroneously given a weeknumber of 53 when it should be 1.

There are a variety of workarounds.

In Excel 2013+ (Excel for Mac 2011+) you can use for the ISO Weeknumber:

isoWeekNumber = WorksheetFunction.isoWeekNum(dt)

For earlier versions, you can test the Monday and adjust it if necessary, or you can write a separate routine.

Answer 4

Calculate ISO year using datepart with bugs workaround:

'Test 2007-12-31 should return W01Y2008

myDate = "2007-12-31"

ISOWeek = DatePart("ww", myDate, vbMonday, vbFirstFourDays)
Week1 = DatePart("ww", myDate, vbMonday, vbFirstFourDays)
Week2 = DatePart("ww", DateAdd("d", 7, myDate), vbMonday, vbFirstFourDays)
ISOYear = DatePart("yyyy", myDate, vbMonday, vbFirstFourDays)
Year1 = DatePart("yyyy", myDate, vbMonday, vbFirstFourDays)
Year2 = DatePart("yyyy", DateAdd("d", 7, myDate), vbMonday, vbFirstFourDays)

If ISOWeek = 53 And DatePart("ww", DateAdd("d", 7, myDate), vbMonday, vbFirstFourDays) = 2 Then
   ISOWeek = 1
End If

if ISOWeek = 1 And DatePart("yyyy", DateAdd("d", 7, myDate), vbMonday, vbFirstFourDays) > ISOYear Then
 ISOYear = ISOYear + 1
End If

MsgBox("W" & ISOWeek & "Y" & ISOYear)

' Result in W01Y2008

Answer 5

So, this is my final and working perfectly version

Public Function IsoWeekNumber(d As Date) As String

    Dim kwtemp As String

    kwtemp = DatePart("ww", d, vbMonday, vbFirstFourDays)

    If Len(kwtemp) = 1 Then kwtemp = "0" & kwtemp

    IsoWeekNumber = kwtemp

End Function

If Application.International(xlMDY) = True Then

HeadlineTemp = Mid(VRHeadline, InStr(VRHeadline, "[") + 1, 10)
HeadlineTemp = Replace(HeadlineTemp, ".", "/")
HeadlineTemp = Mid(HeadlineTemp, 4, 3) & Left(HeadlineTemp, 2) & Right(HeadlineTemp, 5)
VRFirstKW = CDate(HeadlineTemp)
HeadlineTempEndKW = Mid(VRHeadline, InStr(VRHeadline, "]") - 10, 10)
HeadlineTempEndKW = Replace(HeadlineTempEndKW, ".", "/")
HeadlineTempEndKW = Mid(HeadlineTempEndKW, 4, 3) & Left(HeadlineTempEndKW, 2) & Right(HeadlineTempEndKW, 5)
VREndKW = CDate(HeadlineTempEndKW)
VRKW = "KW" & IsoWeekNumber(VRFirstKW) & "-" & IsoWeekNumber(VREndKW) & "/" & Year(VREndKW)

    Else 'don't switch position of the month with days

    HeadlineTemp = Mid(VRHeadline, InStr(VRHeadline, "[") + 1, 10)
    HeadlineTemp = Replace(HeadlineTemp, ".", "/")
    VRFirstKW = CDate(HeadlineTemp)
    HeadlineTempEndKW = Mid(VRHeadline, InStr(VRHeadline, "]") - 10, 10)
    HeadlineTempEndKW = Replace(HeadlineTempEndKW, ".", "/")
    VREndKW = CDate(HeadlineTempEndKW)
    VRKW = "KW" & IsoWeekNumber(VRFirstKW) & "-" & IsoWeekNumber(VREndKW) & "/" & Year(VREndKW)

Answer 6

Be carefull when it comes to week numbers as there are different definitions around. The Excel definition differs from the ISO definition. To get the ISO weeknumber use (copied From http://www.rondebruin.nl/win/s8/win001.htm)

Public Function IsoWeekNumber(d As Date) As Integer
    Dim d2 As Long
    d2 = DateSerial(Year(d - Weekday(d - 1) + 4), 1, 3)
    IsoWeekNumber = Int((d - d2 + Weekday(d2) + 5) / 7)
End Function