更改Calendar1中某一天的底色,为什么点下一个单元格,着色就消失了?

Question

更改Calendar1中某一天的底色,为什么点下一个单元格,着色就消失了?大家帮看看...仅有的分,不够下次补上...谢了.
procedure TDayForm.N1Click(Sender: TObject);
var
    TheText: string;
    x1,y1,x2,y2:integer;
    w,h:integer;
begin
    TheText := Calendar1.CellText[3,3];
    Calendar1.Brush.Color:=clred;
    x1:=Tstringgrid(calendar1).CellRect(3,3).left;
    y1:=Tstringgrid(calendar1).CellRect(3,3).top;
    x2:=Tstringgrid(calendar1).CellRect(3,3).right;
    y2:=Tstringgrid(calendar1).CellRect(3,3).bottom;
    w:=tstringgrid(calendar1).Canvas.textwidth(thetext);
    h:=tstringgrid(calendar1).Canvas.textheight(thetext);
    tstringgrid(calendar1).Canvas.Brush.Color:=clGreen;
    tstringgrid(calendar1).Canvas.TextRect(Tstringgrid(calendar1).CellRect(3,3),
    x1+(x2-x1-w) div 2,y1+(y2-y1-h) div 2,thetext);
end;