Term used to refer to the nebulous 'period' prior to the creation of the Spatial-Temporal Reality. Referring to such as a period or time however is a misnomer as it occurred prior to the actual formation of the Foundational Force commonly known as Time.
It is known that a number of events of a sort took place during the Ancient of Days, including the awakening of the entity known as God, God's creation of Heaven and the Angelic Host, collectively known as the First-Generation Creation, and the darkening of the Angel known as Lucifer Morningstar that led to the Fall War. Such events have been described as truly beyond the understanding of beings who live within and perceive through spatial-temporal means. At one point, the entity known as the Chronicler suggested that any such understanding held of those events is merely the best possible given such limitations, which would also then hold true for any reference thereof.
Also known as the Time Before Time or the Age Before the Ages.
- Wikipedia Article on Ancient of Days