put it in brackets
i.e. end date would have to be [end date] or 'end date'
I would also suggest doing the sorting IN code, so your using the table Default View and then you can give some more custom sorting options if needed. Remove the sort at the end of your statement and do this:
DataTable objtb = new DataTable();
DataView dv = objtb.DefaultView;
dv.Sort = "[start date] asc, [end date] asc";
DGV.DataSource = dv;