WARNING: The following contains spoilers for Adventure Time: Distant Lands - Obsidian, now streaming on HBO Max.
Adventure Time: Distant Lands - Obsidian is in many ways a story about time. After all, its main character is an immortal vampire whose girlfriend is an equally long-lived candy princess. Because of this, the show's producers had a large amount of latitude on when in the Adventure Time timeline the story would take place. A story involving Marceline can take place at any time during the character's lifespan, which was over a thousand years at the show's start. With that said, there's a number of reasons the episode can't take place before the events of the main series.
The previous Distant Lands special, BMO, ended with the surprise twist that the events of the special took place before Adventure Time even started, while Finn and Jake were still children. As such, fans would have already been expecting a similar twist going in. In addition, Obsidian is aimed in part at exploring Marceline and Princess Bubblegum's new relationship. They had dated in the past, but it's their relationship going forward that the special seems interested in diving into. And if you've established it takes place after the events of the main series, there's only one more question -- how long after?
The special opens with a new character, Glassboy, attempting to find Marceline in the Candy Kingdom. As he does so, he runs fairly quickly into Simon Petrikov, no longer the Ice King. This Simon looks fairly young, with brown hair and unlined skin. Seeing that Simon is one of very few humans in the Land of Ooo, he's one of our best barometers for determining how much time has passed. It surely can't have been more than a few years. The rest of the special seems to support this, with a general atmosphere of Bubblegum and Marceline's relationship being comfortably domestic and yet fairly new.
Of course, the story still gets up to plenty of time-hopping shenanigans. Its primary plot is about Marceline and Bubblegum's return to Glass Kingdom, a part of Ooo Marceline had saved from a lava monster centuries earlier. It's also the site of the couple's first breakup, which came in the form of a rage-filled breakup anthem from Marceline. The Vampire Queen's performance of the song in the special cuts between the present and past, showing how Marceline and Bubblegum have changed over the years, despite having not aged. Another set of similar flashbacks occur later in the special when Marceline goes into the desert to find herself, showing her as a young child grieving her mother's death.
After a whole forty minutes of sticking with the previously assumed timeline, the special drives straight off the rails in its final moments, which see Finn arrive to "save the day." This Finn is a fully grown adult, rocking an impressive beard as well as several battle scars in addition to a chest tattoo of Jake, who is notably absent. Instead, we see his granddaughter Bronwyn, who looks mostly the same as the last time she appeared in the show. This in combination with Finn's tattoo seems to imply Jake is dead, which is a little bit of a bummer. It also makes figuring out when the special takes place a little trickier. Finn looks to be decades older than he was in the show's finale, but Simon hasn't aged a day. It's possible his previous ties to Ice King's crown have left him immortal.
Obsidian's final twist makes sense given its themes. It's a story about immortals, whose relationship has been glacially slow in the past, Marceline and Bubblegum's last breakup lasted centuries, and it comes as little surprise that decades passing would still see them acting like a young couple. One of the lyrics in Marceline's heartfelt ballad is even "we'll never grow old together, because you'll never grow old to me." Even as their mortal companions age and pass away, The Bubblegum Princess and Vampire Queen will remain young at heart.
Created by Pendleton Ward, Adventure Time: Distant Lands is available exclusively on HBO Max.