From about.atheism :
A more important basis for defining eternal as timeless is the ancient Greek idea that a perfect god must also be an immutable god. Perfection does not allow for change, but change is a necessary consequence of any person who experiences the changing circumstances of the historical process. According to Greek philosophy, especially that found in the Neoplatonism which would play an important role in the development of Christian theology, the most real being was that which existed perfectly and changelessly beyond the troubles and concerns of our world.
Eternal in the sense of everlasting, on the other hand, presumes a God who is part of and acts within history. Such a god exists through the course of time like other persons and things; however, unlike other persons and things, such a god has no beginning and no end. Arguably, an everlasting god cannot know the details of our future actions and choices without impinging upon our free will. Despite that difficulty, however, the concept of everlasting has tended to be more popular among average believers and even many philosophers because it is easier to comprehend and because it more compatible with the religious experiences and traditions of most people.