I have this code to export to
Use NumberFormat :
Example:
string FRM_EXCEL_FECHA = "dd/MM/yyyy"; // o el formato que deseas:
ws.Cells[i + 2, j + 1].NumberFormat = FRM_EXCEL_FECHA;
ws.Cells[i + 2, j + 1].Value = dataGridView2.Rows[i].Cells[j].Value.ToString();
Give that format to your column
Range rango = (Excel.Range)worksheetobject.Cells[1,1];
rango.EntireColumn.NumberFormat = "DD/MM/YYYY";