Why is a date field being written from a Grid to a database NOT seen as a date field

I am trying to write data from a grid  to a postgresql database .

The datagrid with the data is shown below:


I got an error  saying that a field is not recognised as valid date field
But i know the field is declared  as a date field. See the error screen below


Note that the string GridColumn03  and gridColumn04    are showing as string and
i tried the Convert  statement

My full  Method is shown below

public void CreateJobHistoryForm(DataTable dataSource)
        {
            // New datatable implementation  24-05-2019
            //Place  to process tuples into the database
            //  Save Button Click
            using (NpgsqlConnection conn = new NpgsqlConnection(pgrstring))
            {
                // public.speducation_insert
                using (var command = new NpgsqlCommand("public.spjobhistory_writemasterdetail", conn))
                //using (var command = new NpgsqlCommand("public.speducation_insert", conn))
                {

                    conn.Open();
                    command.CommandType = CommandType.StoredProcedure;

                    //command.Parameters.Add(value: new NpgsqlParameter("staffnox", NpgsqlTypes.NpgsqlDbType.Varchar) { Direction = ParameterDirection.Input, Value = GridVariables.GridColumn01 });
                    //command.Parameters.Add(new NpgsqlParameter("companycodex", NpgsqlTypes.NpgsqlDbType.Varchar) { Direction = ParameterDirection.Input, Value = GridVariables.GridColumn07 });
                    //command.Parameters.Add(new NpgsqlParameter("empidx", NpgsqlTypes.NpgsqlDbType.Integer) { Direction = ParameterDirection.Input, Value = ((int)GridVariables.GridColumn08) });
                    //command.Parameters.Add(new NpgsqlParameter("empeduidx", NpgsqlTypes.NpgsqlDbType.Integer) { Direction = ParameterDirection.InputOutput, Value = ((int)GridVariables.GridColumn09) });

                    NpgsqlParameter staff_no = new NpgsqlParameter("staffnox", NpgsqlTypes.NpgsqlDbType.Varchar) { Direction = ParameterDirection.Input };
                    command.Parameters.Add(staff_no);
                    NpgsqlParameter companyname = new NpgsqlParameter("companynamex", NpgsqlTypes.NpgsqlDbType.Varchar) { Direction = ParameterDirection.Input };
                    command.Parameters.Add(companyname);
                    NpgsqlParameter startdate = new NpgsqlParameter("startdatex", NpgsqlTypes.NpgsqlDbType.Date) { Direction = ParameterDirection.Input };
                    command.Parameters.Add(startdate);
                    NpgsqlParameter enddate = new NpgsqlParameter("enddatex", NpgsqlTypes.NpgsqlDbType.Date) { Direction = ParameterDirection.Input };
                    command.Parameters.Add(enddate);
                    NpgsqlParameter yearsexp = new NpgsqlParameter("yearsexpx", NpgsqlTypes.NpgsqlDbType.Integer) { Direction = ParameterDirection.Input };
                    command.Parameters.Add(yearsexp);
                    NpgsqlParameter designationid = new NpgsqlParameter("designationidx", NpgsqlTypes.NpgsqlDbType.Varchar) { Direction = ParameterDirection.Input };
                    command.Parameters.Add(designationid);
                    NpgsqlParameter companycode = new NpgsqlParameter("companycodex", NpgsqlTypes.NpgsqlDbType.Varchar) { Direction = ParameterDirection.Input };
                    command.Parameters.Add(companycode);
                    NpgsqlParameter empid = new NpgsqlParameter("empidx", NpgsqlTypes.NpgsqlDbType.Integer) { Direction = ParameterDirection.Input };
                    command.Parameters.Add(empid);
                    NpgsqlParameter empjobid = new NpgsqlParameter("empjobidx", NpgsqlTypes.NpgsqlDbType.Integer) { Direction = ParameterDirection.Input };
                    command.Parameters.Add(empjobid);

                    staff_no.Value = GridVariables.GridColumn01;
                    companycode.Value = GridVariables.GridColumn07;
                    empid.Value = GridVariables.GridColumn08;
                    empjobid.Value = GridVariables.GridColumn09;

                    string staffnop = "";
                    string companynamep = "";
                    string startdatep = "";
                    //  Use paratmeters above to test that record is not duplicated twice in the same recordset ( check duplicates )

                    foreach (DataRow row in dataSource.Rows)
                    {
                        if ((staffnop != GridVariables.GridColumn01) && (companynamep != GridVariables.GridColumn02) && (startdatep != GridVariables.GridColumn03))
                        {
                            companyname.Value = GridVariables.GridColumn02;
                            startdate.Value = Convert.ToDateTime(GridVariables.GridColumn03);
                            enddate.Value = Convert.ToDateTime(GridVariables.GridColumn04);
                            yearsexp.Value = Convert.ToInt32(GridVariables.GridColumn05);
                            designationid.Value = GridVariables.GridColumn06;

                            int newvar = 0;
                            //  int newid = Convert.ToInt32(command.ExecuteNonQuery());
                            int newid = Convert.ToInt32(command.ExecuteScalar());
                            newvar = newid;
                            staffnop = GridVariables.GridColumn01;
                            companynamep = GridVariables.GridColumn02;
                            startdatep = GridVariables.GridColumn03;
                        }
                    }
                    return;
                }
            }
        }

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How can i resolve this error

Thanks

Olukay