日期在DELPHI中的运算(50分)

如:我取到一个日期为20080319,如何把他变成2008,然后再通过什么运算可以分别得到200802,200801,200712,200711,200710,200709.

procedure TForm1.Button1Click(Sender: TObject);

var
Present: TDateTime;
Year, Month, Day, Hour, Min, Sec, MSec: Word;
begin
Present:= Now;
DecodeDate(Present, Year, Month, Day);
Label1.Caption := 'Today is Day ' + IntToStr(Day) + ' of Month '
+ IntToStr(Month) + ' of Year ' + IntToStr(Year);
DecodeTime(Present, Hour, Min, Sec, MSec);
Label2.Caption := 'The time is Minute ' + IntToStr(Min) + ' of Hour '
+ IntToStr(Hour);
end;