Object of class DateTime could not be converted to string

nanya ini cara convert nya bagaimana ? di mysql sudah ku coba tipe data date, datetime, varchar, text tapi masih tetep muncul.. masalahnya di cell "Tanggal beli" excel. harus gimana biar sinkron bisa di upload ?

"Object of class DateTime could not be converted to string"

dadw.png

Controler import/upload Excel

public function uploadexcel()
    {
        $config['upload_path'] = './uploads/';
        $config['allowed_types'] = 'xlsx|xls';
        $config['file_name'] = 'doc' . time();
        $this->load->library('upload', $config);
        if ($this->upload->do_upload('importexcel')) {
            $file = $this->upload->data();
            $reader = ReaderEntityFactory::createXLSXReader();

            $reader->open('uploads/' . $file['file_name']);
            foreach ($reader->getSheetIterator() as $sheet) {
                $numRow = 1;
                foreach ($sheet->getRowIterator() as $row) {
                    if ($numRow > 1) {
                        $dataasset = array(
                            'id_category' => $row->getCellAtIndex(1),
                            'no_asset' => $row->getCellAtIndex(2),
                            'name' => $row->getCellAtIndex(3),
                            'description' => $row->getCellAtIndex(4),
                            'merek' => $row->getCellAtIndex(5),
                            'kw' => $row->getCellAtIndex(6),
                            'position' => $row->getCellAtIndex(7),
                            'toonase' => $row->getCellAtIndex(8),
                            'x' => $row->getCellAtIndex(9),
                            'y' => $row->getCellAtIndex(10),
                            'z' => $row->getCellAtIndex(11),
                            'type_spek' => $row->getCellAtIndex(12),
                            'tgl_beli' => $row->getCellAtIndex(13),
                            'harga_beli' => $row->getCellAtIndex(14),
                            'persen' => $row->getCellAtIndex(15),
                            'masa_depresiasi' => $row->getCellAtIndex(16),
                            'asset_kontrol_ketua' => $row->getCellAtIndex(17),
                            'asset_kontrol_wakil' => $row->getCellAtIndex(18),
                            'status' => $row->getCellAtIndex(19),
                        );
                        $this->m_assetdata->import_data($dataasset);
                    }
                    $numRow++;
                }
                $reader->close();
                unlink('uploads/' . $file['file_name']);
                $this->session->set_flashdata('success', 'Import Excel berhasil');
                redirect('asset_data');
            }
        } else {
            echo "Error : " . $this->upload->display_errors();
        }
    }