datetime value update into Database in c# from datagridview

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C#

When I select Date in Datagrid view Column for update into Database Here is shown an Error: The string was not recognized as a valid DateTime. There is a unknown word starting at index 0.
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Here is my Code :

private void DataGridDataInsert()
        {
            DataGridViewRow dgr;
           
            int  i;
            string Pname,amount,inerID ;
            DateTime insertdt;
            //double amount;
           
            for (i = 0; i < (dtProductTray.Rows.Count - 1); i++)
            {
                dgr = dtProductTray.Rows[i];

//Shown Error : in below line:
                insertdt= Convert.ToDateTime(dgr.Cells[2].Value);//The string was not recognized as a valid DateTime. There is a unknown word starting at index 0.
                amount = dgr.Cells[3].Value.ToString();
                inerID =dgr.Cells[0].Value.ToString();
                
                DB_Manager.UpdateData("update datatable set insert_date='" +Convert.ToDateTime( insertdt.ToString("MM/dd/yyyy")) + "',amount=" + amount.ToString() + " where iner_id=" + inerID + " ");
                           }
        }

Plz help