It was exactly what the trailer presents it as: a mostly lighthearted coming-of-age movie that features sort-of time travel caused by recreational self-medication. Like most time travel scenarios, the whole “if I take enough mushrooms I can conjure my future self” thing makes no sense, but the Elliotts acknowledge that they don’t know how it works and let it go.
Bonus: it doesn’t overstay its welcome. A sharp ninety minutes that gives all the story it needs to give and moves on.