Without using any SqlFunction
var grandTotalWork = (from t in db.AssignmentHandymandetails join ad in db.AssignmentDetails on t.assignment_detail_id equals ad.assignment_detail_id where ad.assignment_id == Convert.ToInt32(Label_assignmentId.Text) select new { startTime = t.started_time.Value.TimeOfDay, endTime = t.end_time.Value.TimeOfDay, TotalWorkTime = (t.started_time.Value.TimeOfDay.Duration() - t.end_time.Value.TimeOfDay.Duration()).Duration()});
then you can bind the result in GridView you wish