Makro für deutsche Kalenderwochen

Sub kalenderwoche()
Dim a As Integer, dat As Date
For r = 1 To 365
dat = Cells(r, 1)
a = Int((dat - DateSerial(Year(dat), 1, 1) + _
((WeekDay(DateSerial(Year(dat), 1, 1)) + 1) Mod 7) - 3) / 7) + 1
If a = 0 Then
a = DateSerial(Year(dat) - 1, 12, 31)
ElseIf a = 53 And (WeekDay(DateSerial(Year(dat), 12, 31)) - 1) Mod 7