ISO 8601 week number calculation in LotusScript

weeknum.ls

Function WeekNumber(t As NotesDateTime) As String
  ' Wrapper to work out week numbers of NotesDateTime objects in the 
  ' current time zone
  Dim lst As Variant
  lst = t.LSLocalTime
  WeekNumber = LSWeekNumber(lst)
End Function

Function LSWeekNumber(t As Variant) As String
  ' Returns the week number of a NotesDateTime object
  ' computed in the local time zone according to ISO 8601
  ' which states that week 1 is the week containing the first Thursday
  ' (or, equivalently, the week containing January 4th).
  ' Returns a 7-character string; example: "2010W05"
  ' First, strip any time information and get year, month, day, and day-of-week
  Dim lst As Variant
  lst = Int(t)
  Dim yyyy As Integer
  yyyy = Year(lst)
  Dim mm As Integer
  mm = Month(lst)
  Dim dd As Integer
  dd = Day(lst)
  dow = Weekday(lst)
  ' The first special cases are when the first week of the next year 
  ' protrudes back into this year and we've been asked to compute week number 
  ' for a date right at the end of the year
  If (mm = 12) Then
    If (dd >=29 And dow = 2) Or (dd >= 30 And dow = 3) Or (dd = 31 And dow = 4) Then
      LSWeekNumber = Trim(Str(yyyy)) & "W" & "01"
      Exit Function
    End If
  End If
  ' OK, not a special case, so we have some work to do
  ' Create a date number for the first day of the year
  Dim fdoy As Variant
  fdoy = Datenumber(yyyy, 1, 1)
  ' Work out what day of the week that was
  ' Note that Notes uses Sunday = 1, which is non-standard as per ISO 8601
  Dim fdow As Integer
  fdow = Weekday(fdoy)
  ' Compute offset
  Dim offset As Integer
  If (fdow > 5) Then
    offset = fdow - 9
  Else
    offset = fdow - 2
  End If
  ' Compute week number
  Dim wn As String
  wn = Right("0"&Trim(Str(Int(((lst + offset) - fdoy) / 7) + 1)), 2)
  If (wn = "00") Then
    ' It's the last week number of the previous year,
    ' i.e. the week number it was on December 31st of that year
    ' so call ourselves recursively to compute that
    wn = Right(LSWeekNumber(Datenumber(yyyy-1, 12, 31)), 2)
    LSWeekNumber = Trim(Str(yyyy - 1)) & "W" & wn
    Exit Function
  End If
  LSWeekNumber = Trim(Str(yyyy)) & "W" & wn
End Function

There's a bug at row 30 the correct is LSWeekNumber = Trim( Str( yyyy + 1 )) & "W" & "01" in fact 31/12/2018 return 2019W01

Read ISO 8601: 2018/12/31 IS in the first week of 2019