Function WeekNumber(Optional ByVal vDate As Variant) As Byte
If IsMissing(vDate) Then vDate = Date
WeekNumber = DatePart("ww", vDate, vbMonday, vbFirstFourDays)
End Function
Sub Exemple_Utilisation()
MsgBox WeekNumber(#4/15/2007#)
MsgBox WeekNumber()
End Sub