Ancient of Days (Atik Yomin) is one of the biblical names for God.

In Judaism

This term appears three times in the book of Daniel (7:9, 13, 22), and is used in the sense of God being eternal. In contrast with all earthly kings, God's days are past reckoning.

See also The names of God in Judaism

In Christianity

In Eastern Orthodox Christian hymns and icons, the Ancient of Days is sometimes identified with God the Father, and sometimes with God the Son, or Jesus Christ. As such, an icon 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.

"The Ancient of Days" is also the name of a famous painting by William Blake