THIS YEAR, εφέτος ή φέτος εφ- fr. επ- fr. επί + έτος
YEAR ***έτος (το) [του έτους, τα έτη, των ετών] αρχ. Ελ. Fέτος (F is the old digamma, I do not have the symbol, a v/w sound.) fr. I.E. wet- (year)
LAST YEAR πέρσι ή πέρυσι fr. I.E. peruti fr per-(beyond) + wt wet- (year)
NEXT YEAR του χρόνου χρόνος time or year, unknown origin. My theory is that it comes from Zeus' father, Kronos/ Κρόνος, who ate his kids, because there is a saying in my language that "Time eats it's children" and we are the children of Time
In theory, it's ambiguous, but in practice, it usually isn't.
I suppose one could construct a sentence such as "We will measure the length of the year" or "We will measure the length next year", but otherwise, it should be pretty clear which meaning is meant since one is used like an adverb, the other will be after a noun.