Holy Trinity Orthodox Parma Ohio
This is an icon that surprises some people, and bothers others. 'IC XC' is a Greek abbreviation for Jesus Christ. 'C' is 'Σ', sigma. Jesus is portrayed as an old man. Three lines from Daniel vii, and a passage from Apocalypse i identifies the Ancient of Days with God, the Son of Man, and Judgment. The description in Apocalypse is that of Daniel, and further in Apocalypse He is identified as 'the First and the Last'. As God is the Trinity, any title of one Person is true of all Three. Especially, the Russians identify the Ancient of Days with Jesus.
Now, this church is heavily muralled with iconography. A corridor that leads to the social hall is painted with the Book of Revelation, and the Ancient of Days is shown speaking to the Apostle John.
Here is some of the theology [click]. Also, Thomas Aquinas identifies the Ancient of Days with the Father; the Russian Church with the Son.
The Ancient of Days is He Who is beyond time, Who existed before time, the Trinity (God).
Now, to the non-Orthodox the Ancient of Days is a William Blake watercolor etching that is quite different. Blake painted Urizen (the Demiurge, the Craftsman) creating with a geometer's compass. That is not an icon. It is not Christian art. This is Blake's mythology borrowing from Platonism, Pythagorean thought, and possibly Freemasonry. At one time, this was a common college poster decorating dormitories; and sometimes seen in math books.