Season 7 filmed from September 2016 to February 2017 for a July 2017 premiere
Season 8 will film from October 2017 to April 2018 which would potentially make September 2018 doable. But it's hard to say that without knowing how long the episodes will be or how extensive the special effects are. They might need more time for post production than usual. They may also want to pair the final season of GoT with the launch of a new drama, like Utopia (which starts filming in October, so it probably wouldn't be ready for September 2018)
It'll probably be January 2019. We might see the an episode of GoT going head to head against the Super Bowl
Season 8 will film from October 2017 to April 2018 which would potentially make September 2018 doable. But it's hard to say that without knowing how long the episodes will be or how extensive the special effects are. They might need more time for post production than usual. They may also want to pair the final season of GoT with the launch of a new drama, like Utopia (which starts filming in October, so it probably wouldn't be ready for September 2018)
It'll probably be January 2019. We might see the an episode of GoT going head to head against the Super Bowl