What does it mean for a rational subject to be omniscient (as that word is commonly applied to the divine)? Does it require that a rational subject know all states of affairs (past, present and future) as actual or only as possible? Certain theologians and philosophers have called the traditional view of omniscience into question. For example, Owen Thomas (a contemporary systematic theologian) suggests that God may know all that it is possible to know: not every actual state of affairs. We also have the open theists who believe (I'm generalizing here) that the future is partly open to God and partly closed. In other words, God does not exhaustively know the future.
Similarly with the issue of time. Nicholas Wolterstorff and William Hasker have both maintained that God possibly has his own time-strand. Maybe God is everlasting in the sense that he had no beginning and will have no end. That is to say, God is always temporal, but his temporality differs from ours (See Psalm 90:2).
To be honest, I cannot make sense of a timeless God. It seems to conflict with what we know about agency in general. What does it mean for an agent (a doer) to be timeless? How does thinking, knowing or awareness occur/exist in a timeless sphere of being? This is not to say that God must be temporal if he is agential. I just don't understand how we correlate agency with divine timelessness.
Finally, if God is the ultimate first cause, then how do we explain the concept of atemporal causation? This argument was raised by Stephen T. Davis in Logic and the Nature of God. It has since been discussed in subsequent works of philosophy, yet, I'm not sure that the question has been answered in a satisfactory manner.