Eleonore Stump and Norman Kretzmann argue that God's duration is atemporal. Their joint concept of divine "eternal-simultaneity" is based on Boethius' account of divine eternity (understood as timeless duration) and Einsteinian relativity, which contributes to the simultaneity part of their general thesis.
Nonetheless, even some who advocate divine timelessness have found glaring weaknesses in Stump and Kretzmann's argument. The infirmities constituting this idea evidently are so profound that Stump and Kretzmann have even seen fit to revise their understanding of eternal-simultaneity. It seems that in order to have duration, we need time (tempus) or temporal parts. The concept of "atemporal duration" does not seem to be coherent. Paul Fitzgerald deals with this subject matter in "Stump and Kretzmann on Time and Eternity," The Journal of Philosophy, 82.5. (May 1985): 260-269. He contends that it appears difficult to maintain a belief in atemporal duration without simultaneously proposing (finite) temporal parts for this supposed divine property:
"So we can opt for a doctrine of God's eternality as not involving in se any duration or mode of extension at all (the point rather than the line). Or we get E-duration in God, in which case there are subphases at which distinct particulars of the divine life have their locations and E-durations (or could at least, even if in fact the divine duration is absolutely monotone, as would be suggested by the doctrine of divine immutability)."
See p. 264 of the aforementioned article.