The term 'Ancient of Days' has many different meanings. It appears several times in the Kabbalah, where it is applied to the First of the Sephiroth - Kether. It is also used in connection to Macropospos, who is 'God as he is himself'. Additionally, the term denoted 'the holy ones of the highest', and it is occasionally used in relation to Israel.
In The Divine Names, the term is defined by Dionysius as 'both the Eternity and the Time of all things prior to days and eternity and time.'