Well. Considering that Katma is on the cover I think it's safe to say the cover is placed/set in the past (somewhere between the Wolfman/Wein/Englehart eras is my best guess)
Also, considering this entire run/volume is an examination of all of Green Lantern (and Hal's) 60 year history since the very start that began with Showcase #22 (which has its 60th Anniversary coming up this October 2019 when issue #12 of this series is supposed to be released non-coincidentally I'm sure) just like Morrison's Batman (which he built up to during the 52 weekly series) was an examination of all of Batman (and Bruce's) 75 year history coinciding with Batman's 75th Anniversary of his debut in Detective #27, a period piece cover seems appropriate.