Here's the link I promised. https://dictionary.cambridge.org/grammar/british-grammar/dummy-subjects
You're right, "i år" isn't the subject; it's an adverb in the same way that "perhaps" would be an adverb in the same place.
i dag = today (=this day)
neste dag = (the) next day
i år/dette året = this year
neste år = (the) next year
Not all time expressions can take the preposition "i", in which case you resort to "denne/dette" to express "this".
If you see "i + [definite form]" when dealing with time, the "i" translates to "per" or "an/a" in the meaning of per:
fem ganger i timen = five times per/an hour