I've implemented two functions that convert correctly (according to ISO
8601) from a date to a week (=DateToWeek) and from a week to a date
(=WeekToDate). See either
http://www.pvv.org/~nsaa/ISO8601.html
or
http://www.pvv.org/~nsaa/excel.html#22

Look for
Public Function DateToWeek(ByVal datDate As Date, _
Optional ByVal bytTruncFormat As Byte = 0, _
Optional ByVal bytShortLongFormat As Byte = 0) As String

Public Function WeekToDate(ByVal strWeek As String) As Date