add mês em um datetime

Var
dia,mes,ano : word;
begin
decodedate(ano,dia,mes,data)
mes := mes + 1;
if mes = 13 then
  begin
    mes = 1
    ano = ano + 1
  end
data := strtodate(dia + ´/´ + mes + ´/´ + ano)

{*******************************************************}
{                                                       }
{         Delphi VCL Extensions (RX)                    }
{                                                       }
{         Copyright (c) 1995, 1996 AO ROSNO             }
{         Copyright (c) 1997, 1998 Master-Bank          }
{                                                       }
{*******************************************************}

unit DateUtil;

function IncMonth(ADate: TDateTime; Delta: Integer): TDateTime;
begin
  Result := IncDate(ADate, 0, Delta, 0);
end;

{*******************************************************}
{                                                       }
{         Delphi VCL Extensions (RX)                    }
{                                                       }
{         Copyright (c) 1995, 1996 AO ROSNO             }
{         Copyright (c) 1997, 1998 Master-Bank          }
{                                                       }
{*******************************************************}

unit DateUtil;

function IncMonth(ADate: TDateTime; Delta: Integer): TDateTime;
begin
  Result := IncDate(ADate, 0, Delta, 0);
end;

function IncDate(ADate: TDateTime; Days, Months, Years: Integer): TDateTime;
var
  D, M, Y: Word;
  Day, Month, Year: Longint;
begin
  DecodeDate(ADate, Y, M, D);
  Year := Y; Month := M; Day := D;
  Inc(Year, Years);
  Inc(Year, Months div 12);
  Inc(Month, Months mod 12);
  if Month < 1 then begin
    Inc(Month, 12);
    Dec(Year);
  end
  else if Month > 12 then begin
    Dec(Month, 12);
    Inc(Year);
  end;
  if Day > DaysPerMonth(Year, Month) then Day := DaysPerMonth(Year, Month);
  Result := EncodeDate(Year, Month, Day) + Days + Frac(ADate);
end;