As per Rule 47 of CGST Rules 2017, the invoice need be issued within 30 days form supply of service.
Sec 31(2) says " A registered person supplying taxable services shall, before or after the provision of service but within a prescribed period, issue a tax invoice..."
The prescribe period is 30 days in case of supply of services.
Further, as per Rule 50 of CGST Rules, while receiving advance payment you must issue an "Receipt Voucher" (the particulars are mentioned in said rule).
In your case, as the advance is recived in August and service is likely to be provided in Nov 2017, then you should issue a 'reciept voucher' to party making advance payment and after service is provided, issue a tax invoice. Further, if the rate of GST on said service is not known at the time of receipt of advanec then pay GST @ 18%.