The title "Ancient of Days" has been used as a source of inspiration in art and music, denoting the Creator's aspects of eternity combined with perfection. William Blake's watercolour and relief etching entitled "The Ancient of Days" is one such example.
There are many sources for this term, including:
I beheld till the thrones were cast down, and the Ancient of Days did sit, whose garment was white as snow, and the hair of his head like the pure wool: his throne was like the fiery flame, and his wheels as burning fire.
Whenever Judgment looms and the forehead of the Impatient One is revealed, the Forehead of the Ancient of Ancients is revealed; Judgment subsides and is not executed.
The most powerful effect of this particular Name of God stems from the Jewish mystical book the Zohar, the seminal document of Kabbalah that emerged in 13th century Spain. In the Kabbalah there is mention of the Ancient of Ancients, and the Holy Ancient One - Atika Kadisha, variably interpreted as synonymous with the En Sof, the unmanifested Godhead. The Ancient of Days is the manifestation of the Ancient of Ancients within Creation. It refers to the most primary ("ancient") source of Creation in the Divine Will Keter ("Crown").
In 16th century Lurianic Kabbalah, Atik Yomin is systemised as the uppermost Partzuf (Divine "Countenance/Configuration") in the rectification of the World of Atzilut ("Emanation") after the "Shattering of the sephirot Vessels". Keter of Atzilut acts as the guiding Divine motivation in Creation, developing into two partzufim, Atik Yomin (Ancient of Days) and Arich Anpin ("Long Visage/Infinitely Patient One"). Atik Yomin is the inner partzuf of Keter, synonymous with Divine Delight, that enclothes within and motivates Arich Anpin, the outer partzuf of Keter, synonymous with Divine Will. Arich Anpin is said to extend down all levels of Creation in ever more concealed mode as the Divine substratum of everything. The Zohar goes into great detail describing the White Head of God and ultimately the emanation of its anthropomorphic personality or attributes. In the descending realms explained by Luria, the Gulgalta ("Skull"-Keter Will) within Arich Anpin enclothes the Chesed (Kindness) of Atik Yomin, becoming the origin of the lights of the world of Atzilut; the Mocha Stima'ah ("Concealed Brain"-Chokhmah Wisdom) within Arich Anpin enclothes the Gevurah (Severity) of Atik Yomin, becoming the origin of the vessels of the world of Atzilut. The Dikna ("Beard") of Arich Anpin constricts the infinite light originating from Atik Yomin in 13 channels of rectification to lower, relatively finite reality. The Merkabah text Re’ uyot Yehezkel identifies the Ancient of Days as Metatron.
In Eastern Orthodox Christian hymns and icons, the Ancient of Days is sometimes identified with God the Father or occasionally The Holy Spirit; but most properly, in accordance with Orthodox theology he is identified with God the Son, or Jesus Christ. As such, Eastern Christian art will sometimes portray Jesus Christ as an old man, the Ancient of Days, to show symbolically that he existed from all eternity, and sometimes as a young man to portray him as he was incarnate. This iconography emerged in the 6th century, mostly in the Eastern Empire. It was declared by the Russian Orthodox Church at the Great Synod of Moscow in 1667 that the Ancient of Days was the Son and not the Father.
In the Western Church similar figures usually represent only God the Father. Building his argument upon the Daniel passage, Thomas Aquinas identifies the Ancient of Days as the Person of the Father.
In the hymn "Immortal, Invisible, God only Wise", the last two lines of the first verse read:
Daniel 7:13-14 says, “I kept on beholding in the visions of the night, and, see there! with the clouds of the heavens someone like a son of man happened to be coming; and to the Ancient of Days he gained access, and they brought him up close even before that One. And to him there were given rulership and dignity and kingdom, that the peoples, national groups and languages should all serve even him. His rulership is an indefinitely lasting rulership that will not pass away, and his kingdom one that will not be brought to ruin." The Ancient of Days [God the Father] bestowing upon the Son of Man [God the Son, Jesus] a kingdom and honor would suggest that they are two distinct identities. Among ancient Christian pseudepigrapha, one Book of Enoch states that He who is called “Son of man,” who existed before the worlds were, is seen by Enoch in company with the “Ancient of Days”
In Buddhism, in the Kevattha Sutta (Digha Nikaya 11), the term "ancient of days" is referred to the creator God Brahma,
"“I am the Great Brahma, the Supreme, the Mighty, the All-seeing, the Ruler, the Lord of all, the Controller, the Creator, the Chief of all, appointing to each his place, the Ancient of days, the Father of all that are and are to be.” ."
In Buddhism, although Brahma or "the creator god" is respected, he is not regarded as absolutely omnipotent and all knowing, but someone in samsara who is subject to ignorance, birth and death (over billions of years) and a disciple of the Buddha.