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

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

WorksheetFunction.WeekNum(now, vbMonday)

Answer 2

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.