vba convert week number (and year) to date?

Question

I have a week number in Cell C13 and a year in Cell C14.

I am using the below formula to convert this into the Thursday of

Answer 1

It works quite ok, taking in mind that the first day of the week is Sunday:

Function fnDateFromWeek(ByVal iYear As Integer, ByVal iWeek As Integer, ByVal iWeekDday As Integer)

        fnDateFromWeek = DateSerial(iYear, 1, ((iWeek - 1) * 7) + iWeekDday - Weekday(DateSerial(iYear, 1, 1)) + 1)

End Function

You have to pass which day do you want as third parameter. Thursday is day number 5. Credits to these guys: http://www.dreamincode.net/forums/topic/111464-calculate-date-from-year-weeknr-and-daynumber/

Answer 2

For a specific year, you can do it like this :

DateAdd("ww", WeekNumber - 1, DateSerial(2017, 1, 5))

And to test it :

Debug.Print Format(DateAdd("ww", WeekNumber - 1, DateSerial(YearNumber, 1, 5)), "ffffd d MMM yy")

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If others are looking into this and don't want a Thursday or don't work on 2017 :

1. change the 5 to fit your need!

2. Or use the below function GetDayFromWeekNumber

Code to test it :

Sub test()
    Debug.Print Format(GetDayFromWeekNumber(2017, 1, 4), "ffffd d MMM yyyy")
End Sub

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And the generic function GetDayFromWeekNumber :

Public Function GetDayFromWeekNumber(InYear As Integer, _
                WeekNumber As Integer, _
                Optional DayInWeek1Monday7Sunday As Integer = 1) As Date
    Dim i As Integer: i = 1
    If DayInWeek1Monday7Sunday < 1 Or DayInWeek1Monday7Sunday > 7 Then
        MsgBox "Please input between 1 and 7 for the argument :" & vbCrLf & _
                "DayInWeek1Monday7Sunday!", vbOKOnly + vbCritical
        'Function will return 30/12/1899 if you don't use a good DayInWeek1Monday7Sunday
        Exit Function
    Else
    End If

    Do While Weekday(DateSerial(InYear, 1, i), vbMonday) <> DayInWeek1Monday7Sunday
        i = i + 1
    Loop

    GetDayFromWeekNumber = DateAdd("ww", WeekNumber - 1, DateSerial(InYear, 1, i))
End Function

Answer 3

As an additional option, for those using the ISO weeknumber system, where Week 1 of the year is the first week of the year containing four days (or the week that includes the first Thursday of the calendar year, and the first day of the week is Sunday.

• DOW is an optional argument representing the day of the week. It will default to Thursday and, since we are using the ISO weeknumber system, should always fall within the current year.

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Option Explicit
Function WNtoDate(WN As Long, YR As Long, Optional DOW As Long = 5) As Date
    'DOW:  1=SUN, 2=MON, etc
    Dim DY1 As Date
    Dim Wk1DT1 As Date
    Dim I As Long

DY1 = DateSerial(YR, 1, 1)
'Use ISO weeknumber system
I = DatePart("ww", DY1, vbSunday, vbFirstFourDays)

'Sunday of Week 1
Wk1DT1 = DateAdd("d", -Weekday(DY1), DY1 + 1 + IIf(I > 1, 7, 0))

WNtoDate = DateAdd("ww", WN - 1, Wk1DT1) + DOW - 1

End Function

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