Proposed to derive from Proto-Italic *atnos, from Proto-Indo-European *h₂ét-no-, probably from *h₂et- (“to go”). Cognate with Oscan akno- (“year, holiday, time of offering”); Umbrian acnu (“years”); Gothic *𐌰𐌸𐌽 (*aþn) or *𐌰𐌸𐌽𐍃 (*aþns, “year”), attested in 𐌰𐍄𐌰𐌸𐌽𐌹 (ataþni, “year”); and dialectal Dutch aden (“year”).
For the root, compare Sanskrit अतति (atati, “he wanders, goes”). For a comparable case of *-tn- yielding a geminate -nn- in Latin, see penna (“feather”), from Proto-Italic *petnā.