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interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  ComCtrls;

type
  TForm1 = class(TForm)
    DateTimePicker1: TDateTimePicker;
    procedure DateTimePicker1Enter(Sender: TObject);
    procedure DateTimePicker1Change(Sender: TObject);
  private
    { Private-Deklarationen }
  public
    { Public-Deklarationen }
  end;

var
  Form1: TForm1;
  GlobalDate : TDateTime;
implementation

{$R *.DFM}

procedure TForm1.DateTimePicker1Enter(Sender: TObject);
begin
  GlobalDate := DatetimePicker1.Date;
end;

procedure TForm1.DateTimePicker1Change(Sender: TObject);
begin

  if DateTimePicker1.Date-GlobalDate<-25 then 
     DateTimePicker1.Date := GlobalDate+1
  else if DatetimePicker1.Date-GlobalDate>25 then
     DatetimePicker1.Date := GlobalDate-1;

  GlobalDate := DateTimePicker1.Date;
end;

end.