Error: String was not recognized as a valid DateTime

foreach (GridViewRow grv in grdTrans.Rows)
{
   DateTime startDate;
   DateTime endDate;
   string d1 = grv.Cells[1].Text.Replace("-", "/").Trim();
   startDate = DateTime.Parse(grv.Cells[1].Text);
   endDate = DateTime.Parse(grv.Cells[2].Text);
   String s = "Insert into tbl_Bank_Transaction1 ([TransDate],[ValueDate],[Chqno],[Remark],[amount],[Type],[branch],[bank],[Pin_Transno],[refno],[date],[PStatus],[Status]) values( '" + Convert.ToDateTime(startDate) + "','" + Convert.ToDateTime(endDate) + "','" + grv.Cells[3].Text + "','" + grv.Cells[4].Text + "','" + grv.Cells[5].Text + "','" + grv.Cells[6].Text + "','" + grv.Cells[7].Text + "','" + ddlbank.SelectedItem.Text + "',null,null,'" + DateTime.Now + "','1','1')".ToString ();
   comman.commanclass.ExecInsertQuery(s);
   ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "javascript:alert('Inserted');", true);
}