[[maeyanes]]

Hola...

Otra opción es redefinir el método UpdateActions de la clase TForm:

Código Delphi [-]
TMyForm = class(TForm)
protected
  procedure UpdateActions; override;
end;

implementation

procedure TMyForm.UpdateActions;
var
  CurrentTime: TTime;

begin
  inherited;
  CurrentTime := Now;
  btnEntrada.Enabled := (CurrentTime >= StrToTime('8')) and (CurrentTime <= StrToTime('11:59'));
  btnEntradaComida.Enabled := (CurrentTime >= StrToTime('12')) and (CurrentTime <= StrToTime('13:59'))
end;

Saludos...