time of supply of service

Trex Gaming_67 (243 Points)

31 December 2018  
Mr. X provides service on 1-4 2018 to Mr. Y
The service is taxable on forward
charge basis
Mr. Y waits for Mr. X to issue invoice in the prescribed period(as per rule 47 i.e 30 days from the provision of service) but Mr. X provides invoice on 1-6-2018
And payment was made immediately after receiving invoice 

When to charge GST? (what is the time of supply?)