date_format() expects parameter 1 to be DateTimeInterface, boolean given

I made CRUD laravel by including the date on the form, but I got an error in the controller, and the contents of the controller:

public function update(Request $request, $id)
{
    $suratmasuk= \App\SuratMasuk::find($id);
    $suratmasuk->no_surat=$request->get('no_surat');
    $suratmasuk->no_agenda=$request->get('no_agenda');
    $tgl_surat=date_create($request->get('tgl_surat'));``
    $format = date_format($tgl_surat,"Y-m-d");
    $suratmasuk->tgl_surat = strtotime($format);
    $tgl_terima=date_create($request->get('tgl_terima'));
    $format = date_format($tgl_terima,"Y-m-d");
    $suratmasuk->tgl_terima = strtotime($format);
    $suratmasuk->sumber_surat=$request->get('sumber_surat');
    $suratmasuk->perihal=$request->get('perihal');
    $suratmasuk->keterangan=$request->get('keterangan');
    $suratmasuk->save();
    return redirect('surat-masuk');

}

via Chebli Mohamed