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var
  Year, Month: TTreeNode;
  i,last_i: TDate;

begin
  TreeView1.Items.Clear;
  i := EncodeDate(YearOf(Now),1,1); // 1.1. im aktuellen Jahr
  last_i := 0;
  Year := TreeView1.Items.Add(NIL,FormatDateTime('yyyy',i));
  repeat
    if (MonthOf(i) <> MonthOf(last_i)) then
      Month := TreeView1.Items.AddChild(Year,FormatDateTime('mmmm',i));
    TreeView1.Items.AddChild(Month,FormatDateTime('d. dddd',i));
    last_i := i;
    i := IncDay(i);
  until (YearOf(i) <> YearOf(last_i));
end;