Gstr-1 and gstr-3b filing issue with overdue/unpaid invoices from customer

Hello there! My company is a service provider and one of my customers is a Government Department. When I issued first billing invoice for this customer this department paid and clear its billing payments few months later as per governemnt department's lengthy billing clearance process from its concerned government account treasury authority. For example I had issued the service chagres billing invoice in May 02, 2018 and got the billing payment in July 26, 2018. Now i am totally confused and could you please assist me how to file and what month i have to consider to file invoice billing details in Gstr-1 & Gstr-3B as i generated the billing invoice in the month of May 2018, but got paid in July 2018. I'll be grateful of any valuable help and will be greatly appricated for this.Thanks a lot.