dies.

Back-formed from the accusative diem (at a time when the vowel was still long), from Proto-Italic *djēm, the accusative of *djous, from Proto-Indo-European *dyḗws (“heaven, sky”). The original nominative survives as *diūs in two fossilised phrases: mē diūs fidius (an interjection) and nū diūs tertius (“day before yesterday”, literally “now (is) the third day”). The d in diēs is a puzzle with some suggesting dialect borrowing and others referring to an etymon *diyew- via Lindeman's Law. But note the possible Proto-Italic allophony between *-CjV- and *-CiV-, which may be the cause for this divergence (See WT:AITC). Cognate with Ancient Greek Ζήν (Zḗn), Old Armenian տիւ (tiw, “daytime”), Old Irish día, Welsh dydd, Polish dzień, but not English day, which is a false cognate. The Italic stem was also the source of Iovis, the genitive of Iuppiter and was generally interchangeable with it in earlier times, still shown by the analogical formation Diēspiter.