Time travel has long been considered science fiction, while appearing in both science fiction and fantasy. The truth is, time travel’s grandfather paradoxes, forking plotlines, and shiny, spinning parts—from Hermoine’s Time Turner, to H.G. Wells’ time machine, to wormholes (thank you, Sliders)—grace both genres, posing inevitable challenges to our understanding of the narrative arc.
Meantime, portal narratives are often considered fantasy, and so-named because they send their characters to another location—often a secondary world (like Narnia). But one could, if one were so inclined, argue that the fourth dimension—time—is also a valid option for transit between different places.
With this consideration, time travel and portal narratives are at least related, sharing characteristics like being transported, and returned, to a different dimension or world.