String Was Not Recognized As A Valid Datetime. I Have This Error On My Insert Data.Please Help Me!!!

                DateTime conv = Convert.ToDateTime(e.Row.Cells[3].Text);

                string dat = conv.ToString("dd-MMM-yy");
                e.Row.Cells[3].Text = dat;
                e.Row.Cells[6].Text = dat;

            }
            if (grdv.Rows.Count > 0)
            {
                grdv.HeaderRow.Cells[0].Text = "Edit";
                grdv.HeaderRow.Cells[1].Text = "Attendance Detail Reference";
                grdv.HeaderRow.Cells[2].Text = "Course Detail Reference";
                grdv.HeaderRow.Cells[3].Text = "Date Of Attend";
                grdv.HeaderRow.Cells[4].Text = "Attendance ";
                grdv.HeaderRow.Cells[5].Text = "Reason ";
                grdv.HeaderRow.Cells[6].Text = "Last Updated Date";
                grdv.HeaderRow.Cells[7].Text = "Last Updated By";
                grdv.HeaderRow.Cells[8].Text = "Student id";
}