I was hasty with the previous answer I did not try all possibilities. It seems it was doing the job too good that the backups are being done even before one week has passed.
Anyway, I simplified it and hope this I have it correct this time
Code:
'If at least one week has passed since last backup, do the backup on the first opportunity
dteCounter = dteCounter + 7
IF Date() >= dteCounter Then
BackupData()
End If