You haven't answer to my comments, below code will work for .net 3.5 + with LINQ
DataTable dtbind = new DataTable();
dtbind = objvehicleBAL.GetTaxdetails();
dtbind = dtbind.AsEnumerable()
.Where(r=>DateTime.ParseExact(r.Field<string>("todate"), "dd/MM/yyyy", null) < DateTime.Now.AddDays(15))
.CopyToDataTable();
GVTax.DataSource = dtbind;
GVTax.DataBind();
you can use
r.Field<DateTime>("todate") <DateTime.Now.AddDays(15)
if you have datetime column.