Obtendo o último dia de um mês informado

Function LastDayOfMonth (Data : TDateTime; lSabDom : Boolean) : TDateTime;
var
Ano, Mes, Dia : word;
AuxData : TDateTime;
DiaDaSemana : Integer;
begin
AuxData := FirstDayOfMonth (NextMonth (Data), False) - 1;
if lSabDom Then
begin
DecodeDate (Auxdata, Ano, Mes, Dia);
DiaDaSemana := DayOfWeek (AuxData);
if DiaDaSemana = 1 Then
Dia := Dia - 2
else
if DiaDaSemana = 7 Then
Dec (Dia);
AuxData := EnCodeDate (Ano, Mes, Dia);
end;
LastDayOfMonth := AuxData;
end;