delphi incday 运行显示错误！！求解？

YYB

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    btn1: TButton;
    procedure btn1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.btn1Click(Sender: TObject);
var
  myDate : TDateTime;
begin
  // Set up our date just before the end of the year 2000
  myDate := EncodeDate(2000, 12, 30);
  ShowMessage('myDate = '+DateToStr(myDate));

  // Add 10 days to this date
myDate := IncDay(myDate, 10);
  ShowMessage('myDate + 10 days = '+DateToStr(myDate));

end;


end.
软件提示：[Error] Unit1.pas(35): Undeclared identifier: 'IncDay'
什么原因啊？好纠结啊？弄不懂，网上也查不到。
我在网上下载了一本DELPHI函数参考大全，为什么有些函数不能用啊，也是出现上述的问题？？谢谢了