function TimeBetween(aDateStart, aDateEnd: TDateTime): String;
var
  Hour, Minute, Second: Integer;
begin
  Result := '';
 
  if (aDateStart < aDateEnd) then
    Second := Round(86400 * (aDateEnd - aDateStart))
  else
    Second := Round(86400 * (aDateStart - aDateEnd));
 
  Hour := Second div 3600;
  Second := Second - (Hour * 3600);
  Minute := Second div 60;
  Second := Second - (Minute * 60);
 
  if (Hour > 0) then
  begin
    if (Length(IntToStr(Hour)) > 2) then
      Result := IntToStr(Hour)
    else
      Result := FormatFloat('00', Hour);
  end
  else
    Result := FormatFloat('00', 0);
 
  Result := Result + ':' + FormatFloat('00', Minute) + ':' +
    FormatFloat('00', Second);
end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  LblTime.Caption := TFunctions.TimeBetween(Inicio, Now);
end;