1. TEditBox의 OnChange Event를 이용하시면 됩니다..
   총금액이 표시되는 것이..  edTotal이라면

Procedure Form1.edTotalOnChange(Sender:TObject);
Begin
  Try
    edVAT.Text:= IntToStr(StrToInt(edTotal.Text) Div 10);
  Except
    edVAT.Text:= '';
  End;
End;

2. 날짜 계산은 Turbo Pascal 때 짠 소스라..
  올리고 싶지는 않지만..  급하신거 같아서 올립니다..
  흉하지는 마시기 바랍니다..

Type
    Date_Rec                 = Record
                                     Year, Month, Day, Chack : Integer;
                               End;

Function  DayOfMonth(Year,Month:Integer):Integer;
Begin
     Case Month Of
          1, 3, 5, 7, 8,10,12 : DayOfMonth:=31;
          4, 6, 9,11          : DayOfMonth:=30;
          2                   : If DayOfYear(Year) = 365 then DayOfMonth:=28
                                Else DayOfMonth:=29;
     End;
End;

Procedure IncMonth(Var Date:Date_Rec; Gap:Integer);
Begin
  With Date do
    Begin
      Month:= Month + Gap;
      Year:= Year + ((Month-1) Div 12);
      Month:= ((Month-1) Mod 12) + 1;
    End;
End;

Procedure IncDay(Var Date:Date_Rec; Gap:Integer);
Var
   TempInt : Integer;
Begin
  TempInt:= Gap;
  While TempInt > DayOfMonth(Date.Year, Date.Month) do
    Begin
      TempInt:= TempInt - DayOfMonth(Date.Year, Date.Month);
      IncMonth(Date, 1);
    End;
  Date.Day:= Date.Day + TempInt;
  If Date.Day > DayOfMonth(Date.Year, Date.Month) then
     Begin
       Date.Day:= Date.Day - DayOfMonth(Date.Year, Date.Month);
       IncMonth(Date, 1);
     End;
End;


From 류..