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Oops. Yes it is an adverb here. So the things became easy. We have no instrumental case but only accusative and prepositional.

My guess is that Marlow referred to other words which really have the mentioned cases.



Каждый год / Каждую неделю / Каждое утро я хожу в школу. - Винительный падеж
В будущем году / На будущей...