Bob Becking gives a scholarly overview of the origin and interpretation of "Ancient of Days."
Bob Becking, "Ancient of Days," in Dictionary of Deities and Demons, ed. Karel van der Toom, Bob Becking, and Pieter W. van der Horst, 2nd ed. (Leiden: Brill, 1999), 44-45
The expression then can be rendered as advanced in days' implying that the deity was seen as one 'far gone in years' or 'ancient of days'. The background of the imagery in Dan 7 has been looked for in Canaanite mythology (EMERTON 1958; COLLINS 1977; 1993); in a Mesopotamian text (KVANVIG 1988); and in contemporary Hellenistic Egyptian mythological patterns (VAN HENTEN 1993). The imagery of the Ancient of Days has influenced the throne visions in 1 Enoch.