B.H. Roberts notes that "Ancient of Days" was a title for deity among ancient peoples.
B. H. Roberts, The Seventy's Course in Theology, 5 vols. (Salt Lake City: The Caxton Press, 1910), 3:91
6. The Druids: "We have reason to believe that the Britons inhabited England not long after the days of Noah. We might therefore expect to find resemblances between their religion and the religion of other ancient peoples; and we are not disappointed. There is a striking correspondence between the system of the ancient Britons and those of the Hebrew patriarchs, the Brahmins of India, the Magi of Persia, and the Greek priests. It was one system that was finally conveyed to these different parts of the globe. Take as a single instances of the many points of comparison, their idea of God. Among their names for the supreme God which they had in use before the introduction of Christianity, were terms which have been literally translated, "God," "Distributor," "Governor," "Mysterious One," the "Eternal," "He that pervadeth all things," "the Author of Existence," "the Ancient of Days." These expressive appellations sufficiently indicate their views of the moral character and attributes of God." (The World's Worship—Dobbins—p. 188).