Billing sederhana menggunakan Delphi

pada kesempatan kali ini saya akan mengajak anda untuk sedikit belajar delphi..
kali ini kita akan buat program sederhana yakni billing menggunakan delphi..
ok??
ini listingnya.

unit Unit1;

interface

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

type
TForm1 = class(TForm)
Edit1: TEdit;
Edit2: TEdit;
Label1: TLabel;
Label2: TLabel;
Edit3: TEdit;
Label3: TLabel;
Edit4: TEdit;
Label4: TLabel;
Button1: TButton;
Button2: TButton;
Button3: TButton;
Timer1: TTimer;
procedure Timer1Timer(Sender: TObject);
procedure Button1Click(Sender: TObject);
procedure FormCreate(Sender: TObject);
procedure Button2Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form1: TForm1;
Tarif : currency;
detik : integer;
implementation

{$R *.dfm}

procedure TForm1.Timer1Timer(Sender: TObject);
begin
edit1.text := timetostr(time);
detik := detik + 1;
if detik = 60 then
begin
tarif := tarif + 20;
detik :=0;
end;
edit4.Text := currtostr(tarif);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
edit2.Text := timetostr(time);
button2.Enabled := true;
button1.Enabled := false;
tarif := 20;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
button2.Enabled := false;

end;

procedure TForm1.Button2Click(Sender: TObject);
begin
edit3.Text := timetostr(time);
button2.Enabled := false;
end;

end.

gimana? mudahkan????