Функция DateSer

DecodeDate(Date,y,m,d);
NewDate:=DateSer(y-4,m+254,d+1234);

function DateSer(y, m, d: Integer): TDateTime;
const
  mj: array[1..12] of Integer = (31, 28, 31, 30, 31, 30, 31,
    31, 30, 31, 30, 31);
var
  add: Integer;
begin
  while (true) do
  begin
    y := y + (m - 1) div 12;
    m := (m - 1) mod 12 + 1;
    if m <= 0 then
    begin
      Inc(m, 12);
      Dec(y);
    end;
    if ((y mod 4 = 0) and
      ((y mod 100 <> 0) or (y mod 400 = 0)))
      and (m = 2) then
      add := 1 //дополнительный день в феврале
    else
      add := 0;
    if (d > 0) and (d <= (mj[m] + add)) then
      break;
    if d > 0 then
    begin
      Dec(d, mj[m] + add);
      Inc(m);
    end
    else
    begin
      Inc(d, mj[m] + add);
      Dec(m);
    end;
  end;
  Result := EncodeDate(y, m, d);
end;