It is an accident of sensation or motion. Time, to Epicurus, is an accident of an accident. The world we experience is a world which is ever changing. But there is something back of it that does not change. To Epicurus, this unchanging principle is the atoms.
Time is devoid of the essential reality of the atom or the Void.
The essential reality of the universe is something which is not accident, and which is timeless. Transition, changingness is not a character of the really real. For both Epicurus and the idealists, if you make endurance fundamental, then time is illusory. If reality is that which endures, then time can be only appearance. There is the creation of the enduring substance.