Sorry,I meant the difference between to consecutive dates must not be more than 20 days. The difference between 13-Sep & 20-sep, 20-sep & 27-sep, 27-Sep & 4-oct is less than 20 days. so, they belong to one group. The difference between 4 oct & 24-No is more than 20 days, so the next group starts from 24 of Nov and so on